Usually you can tell if your Digimon has dark evolved because...well sometimes they shout it out during the evolution process.

The only other way to tell is to guess really. If you've seen them evolve once before, and what they turned into didn't match up...

Also a pretty good give away is if the new form they take looks decidedly scary and evil but with some Digimon that's hard to gauge.

What did Labramon change into?

I spend alot of time reading, watching people and keeping up to date on the posts people leave...you'd be amazed how much you can learn.

And knowledge is the greatest power you know.