If straight out amphetamine your best measure of whether it is good is in its effects. However, speed is notoriously cut to death all over the civilised world so your harm-reduction approach would be as follows:
1. Subject a small sample of your supposed amphetamine to a reagent test. Reagents are easy enough to obtain and quite cheap in most places. However it will tell you if the substance has amphetamines in it, but not the purity.
2. Acetone wash your deal regardless of what your dealer says he has already done. Re-washing it does not do any harm. It will greatly purify out common cuts but there are insoluble cuts that can pass through an acetone wash.
3. If you are shooting it, you may see remaining cuts when you attempt to dissolve it. Filter heavily if you do. Filter anyway.
4. If you are snorting it, by this point there’s likely nothing seriously harmful (except amphetamine of course) left in it. So have a go and see what it does to you. If you think you have near pure amphetamine sulfate after all the above then be very cautious about how much you take. Street speed is very often only 10-20 % pure and a point or two will still light an infrequent user on fire.
Snort a serve about the size of a match head. If it’s pure, you’ll be chewing your tongue off and speaking like a race caller in no time at all.