I've never heard of, seen or bought actual bona fide laced weed that wasn't sold as being laced. The only thing I can think of that would be particularly worrisome (aside from deadly poison or other such things, obviously) would be PCP. I don't know about pricing of that, but I don't think it's cheap enough for anyone to want to give it away to people that don't even want it rather than sell it to people who do.
Coke could be used, but it won't be very effective. You should damn well notice if there's enough coke on your bud to do anything, since burning it is inefficient. Crack is another story, but, again, you should notice, and if you buy herb from someone stupid and mean enough to lace your weed with crack and not tell you, you need a new source.
In general, I'd say it would be very hard not to tell if your herb was laced. For the most part, this 'risk' is made up by the DEA et al to scare people. It doesn't actually happen too often (as I said, never even heard of a specific incident of it happening, and I've known a lot of tokers for a long time)
