Aquarium Pharmaceuticals KH/GH kit is how I do it. Unfortunately, all such measurements measure total alkalinity, not alkalinity due to carbonates. So, if there are a lot of other anions in the water, besides carbonates and bicarbonates, the KH you measure gives a too high reading of ppm CO2.
Well, sometimes it does, sometimes it is accurate.
Hard to know the difference, hence some other folks have suggested the 1.0pH drop from the normal pH measured from a small source of tank water that's equilibrated with the air after 24 hours.
It's actually simpler in some respects.
I have not looked into it critically, but it does seem to work well and gets around the total vs bicarb alk.
Most test kits are fairly accurate for what they do.