I always just gave the battery a good charge, then installed it, connected a voltmeter to it, and used the starter as a load. Watch the voltage drop as the starter turns the motor over. If it quickly drops below 10 volts, the battery is no good.
Before purchasing my VN750 I mined this forum for the pros and cons, know issues and general information. Unanimous was the consensus that a strong AGM type battery was the best solution for many of the bikes starting problems (especially when warm). Many thanks to everyone here! So, the first thing I did when I purchased my VN750 was to replace the O.E. wet battery with a good AGM battery (Yuasa brand). That solved the endless crank to start the engine (especially when the engine was warm). Since then I've added Iridium spark plugs and K&N air filters (again thanks to the recommendations and help from this forum). Near instant starts now when the engine is warm and maybe up to 3 cranks when cold. Going on 4 years now and everything is still going strong, no starting issues, no performance issues and no battery issues. I am a convert that a really good AGM battery is a requirement for this bike. So, I would say replace your current battery if; it is more than a few years old, China made, not AGM, completely drained in the past, suspect in any way, etc.I know that a very vigorous crank is required to start these bikes. I generally use my 100 amp load tester to test my batteries. However I'm thinking that would melt our little batteries. What type of load should I test with?
Your best bet is to take it to any auto parts store for FREE testing. They can compare the CCA's of the battery to what it is rated at.