I found the specs on those megatron batteries here:
http://www.interstatebatteries.com/c...egatron2_f.asp
Although it's labeled funny, I think the RC3 column is the Amp/Hr rating. (you'll see it's a pretty wide range)
For some reason marine batteries are the only ones I usually see A/hr ratings on. (usually right on the battery)
Need one of these to test capacity:
http://www.harborfreight.com/100-amp...&hft_adv=10013