In my case I could tell from the initial rotation speed that the starting speed was slow but I had made multiple attempts (foolishly) without success.   Normally, we should have a voltmeter reference prior to start as a guide.   Below the threshold means charging the batteries or aux power subject to the plane type.   On a regular basis, pulling the batteries and doing a load test/capacity test to determine if they meet [80%] of their stated capacity drawing down at C/10 amp rate as suggested by Concorde and others will weed out early battery problems.   But, I think the best and easiest daily test is simply reading voltage prior to start.  That should give you an approximate charge level.