How do you know if you have stripped the seasoning from a cast iron pan? Is there signs - other than food sticking?

I've been looking into getting a cast iron skillet. As far as my research goes, the older the better and a Griswold is considered a good quality. But these of course either can be found at e-bay/garage sales/ flea markets etc.

Btw, can you cook eggs in them? or should those be done in an aluminium pan.

Thanks