Dean, TQ60 and YC1 gave great answers. I have one of the Harbor Freight load testers...they're about $20.00. It's not fancy and as discussed, will probably only tell you if the battery is toast. For the price, I find it handy to keep in my tool box at home.
One of the first things I learned as a new mechanic out of high school....you CAN'T load test a dead battery. A battery should be fully charged before testing!
Don & Mary
2014 Newmar Dutch Star - All Electric - 450 ISL
2016 GMC Canyon 4WD Crew Cab