Here's a great video on testing a battery using the proper battery tester . There are many different battery testing models and they all work, all you want to accomplish is to put a load on the battery and read the voltage level. You need to know the CCA [cold cranking amp] rating labeled on top of the battery, then draw 50% of that off the battery with your tester for 10 - 15 seconds [the tester has a carbon pile that absorbs the heat from the heavy draw].
A general rule of thumb I use is a battery voltage drop of no less than 9.6 Volts after a 10 second load of 50% of the Cold Cranking Amp Rating. One important factor is the battery must be fully charged before testing, preferably around 12.6 Volts.
A battery tester is affordable but if you don't own one & you're not sure of the condition of your battery then a quick load test at a repair shop is worth it. A reputable shop won't charge you for the test if you buy a battery from them [since the test only takes less than a minute from start to finish].