blarrr this is a tricky one...as most of the time your dealer would tell you if it was laced or not.
Some signs may be in the harshness of the smoke...you should be able to tell within the first puff .. if the pot you're smoking is .. "different". Look at it, smell it... does it look and smell like other pot you've gotten? Is it wet? Danke?
Remember, weed can smell kinda funky if the grower hasn't flushed properly before harvest.. especially with hydro.

I think when it comes down to it, there's no "sure fire sign" that your weed is laced.. just general experience and reliability of your dealer