If it was a meth lab you would know. There would be a chemical (solvent) smell that is terrible and dangerous.
Are the windows blacked out?
is there excessive trash such as duct tape rolls, coffee filters red stains, large number of over the counter drug containers and/or empty blister packs or HEET or other type of fuel containers or any of the following:
Plastic tubing, mason jars, empty pill bottles, starter fluid, rock salt, lithium batteries, peroxide? Or do they take large trash containers to the dump, versus leaving the garbage outside to be picked up?
Do they have a large number of clear containers; clean or dirty?
Are the walls extremely discolored? Might look like brownish stains.
If so, head for the hills or light a match
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Normally, any person who is halfway intelligent will not smoke marijuana in a meth lab. Then again, most meth producers start using the stuff in a few months, and that and the chemicals deteriorate their brain, so they might decide to light up a doobie some afternoon in the lab, which will be the last one they light up.
As for marijuana, it has a distinct, clean, sharp, pungent smell. However, there are some types that smell similar to a lemon tree or spice. Some low-grade product may have an ammonia smell.
If it smells like just cut grass, it is not cured properly, but might still be marijuana versus cut lawn grass.
Marijuana users love to munch on food all the time. In addition, they can appear dizzy and walk weird, act silly and giggle all the time, have red, bloodshot eyes, and forget things that recently happened. After the affects wear down a bit, they normally get very sleepy.
Being Hippie-type chicks, they may have some strange candles or other types of "things" to make the place smell weird, but not illegal.