I definitely would not get my mouth near the battery acid
(I know you didn't plan to do that
I typically use my battery hydrometer, which is used to test for specific gravity. It has a bulb on the end and requires you to suck up the liquid to test. I just wouldn't replace the acid back into the cell. Take out enough to get to the level you need. A turkey baster will work, too.
You can subsequently use the hydrometer to test the battery charge (A hydrometer reading of 1.277 or greater indicates full charge for Trojan batteries) and to make sure there is no variation between the cells. A specific gravity variation of more than 0.050 between cells in a battery means the battery should be equalized.