It's a common question when you knit from a published pattern.
And even me, a (ahem!) well-seasoned knitter, will try to skip knitting a gauge swatch whenever possible. But here are FOUR scenarios where you might not want to skip this important step (sorry!):
ONE: When the finished size matters
The biggest lesson I've learned from past projects gone wrong is that if you really want a finished project to fit correctly, take the time to do the gauge swatch! And I don't mean CO and measure after you've knit a few rows. This is a situation where you will be spending HOURS lovingly knitting a sweater, vest, or what have you, for yourself or a deserving friend or relative. If the end result does not fit - how annoying would that be? To give yourself the best assurance that your finished measurements will match what the pattern has intended, follow these steps to properly swatch: For example, it'd be a real shame if this slim-fitting cardigan was knit up in the wrong gauge - resulting in a cardigan that's too tight or baggy!
1) With the suggested yarn (or a similar yarn in weight and look and feel if you are substituting) and suggested needle size, knit up a swatch that is at least 4" x 4" in the specified stitch pattern that the gauge is given in.
2) BO the swatch and then block it however you will plan to block the finished item. You might even wash it if it's to be a often washed item.
3) After it's fully dry, unpin and measure it on a flat surface. A tape measure, ruler, or special gauge measuring tools (like the 2 shown here) are all fine. You can use sewing pins to mark the 0" point and then the 3", 4", 5", or more point (choose whichever of those inch measurements lands most precisely between 2 sts, rather than in the middle of one since trying to count partial sts is too inaccurate).
4) If you have more sts per inch than the pattern's stated gauge, re-swatch with a bigger needle size. If you have less sts per inch than the pattern's stated gauge, re-swatch with a smaller needle size. Repeat these last 4 steps until you match gauge.
5) At any point, if you don't like how the swatch is looking (at gauge or even before you get gauge) you might decide to try a different yarn. Swatching gives you the chance to test out the yarn with the stitch pattern and at the gauge needed by the pattern.
6) Once you've figured out what needle size will give you the exact gauge needed by the pattern, you can proceed. Hint: I will still measure as I go to double check that my gauge is good . . . it's never too late to turn back if you see a big problem!
REMEMBER - even being a half stitch off can make enough of a difference in the finished size (esp. for a fitted sweater) that you won't want to skip this step. For example, if the sweater is 4 sts to the inch, but you are knitting at 3.5 sts to the inch. . . over a 40" bust, that's say 160 sts. With the slightly off gauge of 3.5, those 160 sts at the bust now measures 45 ¾", which turns a fitted sweater into a baggy one.
7) Matching the pattern's exact row gauge is generally not as important as getting the right stitch gauge, but it does depend on the pattern. Needing to work a few extra or less rows here and there won't matter for most patterns, but read through your pattern and you might decide it matter enough to change yarns.
TWO: When you are substituting a yarn
This is probably the most common reason to knit a swatch. You'll be able to check your gauge AND test if you like the look and feel of the yarn for the pattern. Different textures will result if your yarn weight is a little different, if the yarn content is different, if you are using a smooth yarn vs. a textured one, etc. Swatching is the BEST way to decide if you like your yarn choice before you've knit (or purchased!) a bunch.
THREE: When you might not have enough yarn
Yardage estimates in patterns are BASED on the stated gauge! Particularly for patterns that are "one-ball" patterns, you can't expect the yardage estimate to be accurate if you are knitting at a different gauge. For example, if the pattern gauge is 4 sts per inch and you are knitting at 3.5 sts per inch, you will end up with a larger scarf (or whatever) than intended in the pattern. You'll definitely use up more yards of the yarns to finish the instructions, so plan accordingly. Many published patterns will pad their yardage estimates 10-15%, but that is not an industry standard you can rely on. PLUS, yardage estimates on ball bands are estimates based on the weight of the ball or hank, so there's always some room for error there too.
FOUR: When there are new stitch patterns or techniques in the pattern
Challenging yourself with a new and interesting stitch pattern with this new project? Swatching in that pattern will allow you to learn it before you work it in the actual project. Maybe you'll discover that it's too easy (and therefore "boring") or too hard (which means frustration and/or ripping out rows later). Great things to discover while swatching vs. after you've already started the project!
Hope the above has been helpful. While I'm with you in wanting to skip the step of checking gauge, I'm too aware of the risks, so I almost always swatch first!
FYI - All of my SweaterBabe patterns state the gauge on the pattern sell pages. I find that it helps my customers to select their yarns for my patterns.
OTHER RANDOM TIPS ON GAUGE:
Why is it better to measure across 3 or 4+" vs. 1 or 2"? Less room for error and/or slight inconsistency in knitting tension or stitches. If you measure across just 1" of sts, you may not get an accurate measure that would match if you measured 1" elsewhere. But measuring over 4" or more of sts will average out all those slight variations.
Don't measure too close to the CO, BO, or the edges. How loosely or tightly you CO or BO could skew your measurement, so measure in the middle for best results. This is also why you don't measure when sts are still on the needle, as how they are spread out or bunched up on your needle can skew your measurements easily (plus, you're supposed to block the swatch first!)
Pattern gauges will not always be to the 4" measure. . . sometimes, a true gauge will be something like 23 sts = 5", or 4.6 sts = 1". This does not convert nicely to a 4" measure, as it would have to say 18.4 sts = 4". How does one measure 18.4 sts??
Try to swatch using the actual needles you'll use for knitting the project. For some people, their knitting tension (and therefore their gauge) will vary on bamboo needles vs. ultra smooth aluminum, etc. Even working in the round vs. rows can change your gauge a little.