Just cause its hot outside, it wont mean the battery is even warm enough to take full regen, so no you may not get feedback that you dont have "full" regen. There are regen dots on the display, but there isnt anything that is going to tell you battery temperature.
I am guessing that you may be driving cars that had been sitting a bit. It takes quite a while for a battery to warm up from driving, too. It isnt "a few minutes / miles". When I drive into work, which is just under 40 miles, it can take close to 30 miles, driven at freeway speeds of 80 ish MPH before I have full regen, if my car sat without charging the night before.
I am "fairly sure" that differences in regen would be because of all the side stuff, but lets spin it forward and say it isnt. There wouldnt be anything you could do except buy a used car and test it ( and request that the person had just charged it right before your test drive). You dont get to test drive the car you are buying, if its brand new, and even if you did, unless others had driven it before you, or they had JUST finished charging it for you, the regen is going to feel different.