You can usually tell if it is laced just by looking at it. It will not have a regular decent weed smell. Laced weed is generally schwag or oregano all bound together by some contaminents. Sometimes retarted people will spray weed killer on it to try and get people "addicted" to it, when in fact it just gives a headache and a bad non-cannabis like high. These are usually the hip-hop gang banger wannabees who probably don't even smoke weed and should not even touch a blunt! They are the crack-heads you need to be careful of.

More importantly, pay attention to the prices and names they give it. Laced weed costs more to lace, so they will charge more for it. In the long run, it doesn't really make sense to lace weed. Drug dealers often use street names for their product and laced weed is usually called "wet" in which it is soaked with PCP or embalming fluid. Sometimes they will lace it with coke (Chronic), and I have even heard of them lacing it will opium and crack!!

Just use common sense and you will usually know if its laced. Stay away from laced weed though. Although pure cannabis cannot kill you, laced weed could.