
I had no means of knowing with mine other than when in pain, my hounds tend to 'shake'. And I've not ever seen that. Any 'pain' however could be down to not being able to mate when they are hard-wired to find a mate, when ready and that may be why the pacing. I certainly don't think they suffer with cramps in the same way a human woman can.
My reason for spaying, usually when retired, was mainly so there wasn't the need to confine them twice a year for 3+ weeks, with the added disruption to the others, and I always had in mind the fact that it was more likely they'd develop a female cancer if left entire - much as since mine were usually over 7 when spayed, that risk was pretty much going to still be there. Plus the risk of a pyometra.