To check your gauge in the round, make a swatch on Magic Loop or DPNs and hold your gauge-checker over the stitches. Count how many stitches are in one inch.
Why It's Important To Check Your Gauge In The Round
Your gauge on flat knitting may be different than your gauge when you knit in the round, even when using the same needles and yarn.
For one thing, you'll be doing only knit stitches (probably) when you knit in the round, whereas when you knit flat you will have a row of purl stitches every other row. Since many people purl looser than they knit, this could definitely affect your gauge.
It makes sense to knit your gauge swatch in the same style that you will be knitting the garment. For instance, don't assume that your gauge will be the same on different brand needles of the same size.
Lastly, it follows that if you are going to block your final knitted piece (and you probably should), you should block your gauge swatch as well before you check your gauge. You can do it quickly, but it should be done. See How to block your knitting.
Added: Vertical (Row) Gauge
KnitFreedom student Hilary asked what the vertical gauge is for. The vertical gauge is to check your row gauge - how many rows you get in 1 inch.
Oftentimes the row gauge is less important because you can do more rows to achieve your desired length. For example, if your sweater pattern says, "knit to 20 inches from cast-on edge," you can easily do that without having to worry if that's 370 rows or exactly how many rows it is.
On some patterns it is important and that's when you would check your row gauge using the gauge-checker.