In pretty well all trout and char the females will have a shorter, rounder head and colours tend to be more drab.  Though it's probably least noticeable in lakers.  Personally, I wouldn't worry about it.  If you catch and keep that same fish in the spring - it won't have eggs, but it still won't be spawning come fall.  Good luck, I'll be chasing brookies some where in the park this weekend.

Sorry I'm a little late to reply! But one of the best ways to tell the gender of a mature lake trout is to look for a protrusion from their anus.
If it protrudes (which will be rather slightly, maybe a centimeter) it means that the lake trout has laid eggs and is thus a female. If there is no protrusion it means its either a male lake trout, or an immature female who has not yet laid eggs.