This happened to a friend of mine.. here's the story:
My friend was suspicious about why he was craving more weed.. his dealer actually admitted he was lacing it with heroin and shit. Of course.. my friend never saw him again for that.

It's simple... because it is a liquid, you just take an eye dropper worth and drop it on a dime worth of weed and it will dry into the weed. An eye dropper worth is nothing to bitch about. You can take an eye drop worth from a heroin vial and still sell the heroin for the same price without anyone knowing it weighs less.

Indeed you cannot use coke as it will just fall to the bottom of the bag. But liquid drugs will work. There's probably a way to lace weed with powder by mixing it with a liquid and drying it into the weed of course. So that's probably how it's done.
That's why it's hard to spot laced weed, but you can always smell it for anything weird.