Ask the dealer how much a 20 dollar chunk of tar weighs, buy a scale, weigh said chunk when you get home. If it doesn't weigh out to what the dealer said its supposed to weigh, you got ripped off. Furthermore, its not all about weight. If it takes more than it usual takes to get you high than you bought low quality dope - but there's nothing you can do about that. If you come back telling a dealer his dope wasn't good and you want a refund or a good deal next time, you'd better have a REALLY fuckin chill dealer. If your shit weighs less than its supposed to, he's gotta be at least pretty chill to hook you back up.
Again, i don't know how tar works or how chill the dealers are who sell it as im from New Jersey - we don't have tar. But when i would buy bags of powder dope, if i got a bundle and it wasn't a full 10 bags my dealer would hit me back with the missing bags next time i went down. If i got really shitty dope he would sometimes throw me free ones too next time. But again, i don't know how shit works for you. For me i don't really have to cop when i would go to the hood, except for when i needed a new dealer for whatever reason. Once you find a good dealer, you have his phone number and build a relationship with him so he is more inclined to hook you up and hit you back for shit like that happening. From what i have heard, if you cop dope in the hood than most likely you go to a "spot" and there will be dealers there, but you don't get their phone number or know them very well or talk to them really at all, you just walk up and buy your shit and leave. I have a feeling that kind of dealer will just tell you to fuck off.
Either way now im just rambling. In shot, buy a scale, ask how much a 20 is supposed to weigh, and when you get home weigh it. If it's short you got ripped off.