See my Ventilation page (link in .sig below) for a lot more info on the subject
... but, as far as signs, I would say the big three (in no particular order b/c importance varies with the season) are
-- if coop smells "strong" when you go out there in the morning and open the door, you need more ventilation.
-- if there is condensate or frost forming on the windows etc overnight, you probably need more ventilation. I say "probably" because in extremely cold climates, frosted windows may be inevitable because of the huge temperature difference in vs out and may not reflect a real ventilation problem. But if you can walk out to the coop with bare flesh exposed for more than a minute or two, then if there is much condensate on your windows you need more ventilation. (Condensation/frost on coop ceiling or walls means a severe ventilation deficiency, EXCEPT that in a drippy ceiling may also require insulation in cold climates)
-- if the coop is still uncomfortably hot, like quite noticeably hotter than outdoors, an hour after the sun goes down in the summer, it needs a lot more ventilation.