Don't be silly and buy them off strangers tho!
Possibly the soundest piece of advice on this thread. As powder is easily tampered with, trust is everything.
Powder is also frequently sold in gram weights. It may prove more of an advantage buying it in this form, but there's never a guarantee. It can also be hard to pick a cut if the MDMA itself is not really clean. It's common to see an inconsistent colour in a slightly impure MDMA product, making a cut harder to pick.
If you are really fussy, then examine the powder with a magnifying glass (a printers lens is great). A cut is often more visible this way.
If the MDMA powder has been gas crystallised and washed carefully, it should resemble a fine, white or slightly off white, fluffy powder of fairly even consistency. Those who prepare their own personal caps from powder say that the larger half of a small gel cap will hold ~120mg if filled to the brim.
However, MDMA crystallised with pool acid will be chunky and probably an inconsistent cream to brown in colour. It will tend to form lumps which weigh more. 120mg of this material may only be the equivalent of ~2-3 match heads.
Common cuts are milk and custard powder as they can appear similar to the MDMA, depending on it's colour. These cuts can usually be identified by taste.
If in doubt with MDMA powder, start with a small amount. MDMA powder is normally absorbed fairly quickly so it normally won't take 3/4 hour before you know whether the dose was right for you. However, individual variations in absorption are common.
Keep in mind that apart from absorption-plasma peaks, which often differ between pills and powder, chemically it's exactly the same stuff. Know your limits and keep track of what amounts you're consuming especially if you're doing lines
and dropping.