Other - Buying (newbie)

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SWIM wants to try out heroin for the first time. But she'll need to buy it first. She knows where to buy it but is not sure if she'll be ripped. So,
1) How will she tell if she's getting the real thing?
2) Should 2 matchstick head's equivalent be good enough or will she need more (she'll be smoking, not snorting or injecting)?
3) any other useful advice?

SWIM has no junkie friends so she can't get any help for buying. She has a lot of experience in buying weed but not in heroin.