A case gauge will verify that a case or cartridge is dimensionally within specs so that jams and malfunctions don't occur. If a case is too large, it won't fit the gauge and may not fit your chamber either. There are a number of gauges on the market. The only one I'm familiar with is the one I use, a JP Enterprises gauge. I think it cost around $25.00.

Some may tell you a gauge is not needed, but if 100% reliability from gun and ammo is a concern, you need one, particularly if you ever use handloaded ammunition.