Well naturally it's pretty hard to tell if it's laced weed or ust a killer strain, it could be it's laced with god awful chemicals that do effect you but are a few quid a kilo etc. I've only ever once had weed that made me wonder what the hell i was smoking, a whole ground of us were out in the fields and shared a small joint between us, next thing you know noone can do anything but keep moving, just fecking running around the field, jumping up and down, not through a feeling of elation etc, just the feeling that there was so much energy you had to keep moving.
Other than that i've bought weed plenty of times laced although we'd ust say sprayed, same thing though.