Have a look at the FAQ for this forum. Its quite easy using a test kit to check for the presense of DXM or MDMA.
eg on marquis reagent, DXM will smoke and go a murky grey to black, whilst MDMA will go purple to black (and fizz/smoke sometimes on a purer sample) the two reactions are completely different and impossible to misread.
Since you live in NA you might want to consider getting a Dancesafe Mecke kit, as this gives a very definite result for DXM that can't be confused with MDMA.
This kit is really only useful for differentiating between these two compounds, as it is not so discriminating between others.