Sometimes you can suppose it, when they look very dull, faded and suddenly the other day teh colour are looking much more brighter!! So then they did shed!
But I agree with herperboy, I find it very hard to give any exact information in which time space they shed, because most of the time I guess we won't notice that!!!
Once I could observe it ... was really funny!
If they didn't well shed you can as well notice that they did shed, when you can regonize some rests of old skin on them, but this isn't a good sign and means mainly that they are kept to try or maybe with less vitamins ...
but other signs are hardly to see, so if you wonna know it exactlier it's maybe better to keep some oeduras, they don't eat their skin and you will always see old skin laying around .... :lol:

(Sorry, just kiding ...)
Best Regards
Peter