Generally, If there is any type of unnatural smell, there may be reason to suspect. Now, that doesn't mean everything, because sometimes bud just smells kinda odd. There's even bud with a really strong Ammonia smell called "Cat Piss"! But even then, one can detect the subtle undercurrent of that piny, skunky sort of marijuana smell. Like Captain Hanks said, getting your dealer to smoke with you is a great way to tell as well.

Also, you can try to "hot knife" a little bit of the bud. If there's anything odd like it combusting quickly, sizzling, etc., then there may be reason for you to suspect that it could be laced.

That being said, the lacing of marijuana is very uncommon.