Molecular “fingerprint”
Emily Lakdawalla over at The Planetary Society Weblog covered this question on Sunday during a live web chat from JPL.
According to one of the Phoenix mission scientists, the hydrazine used by the lander has been highly filtered to produce an especially pure form.
This is then sampled by a gas chromatograph mass spectrometer to derive a molecular “fingerprint”.
When Phoenix analyzes samples with it's own gas chromatograph, (Which I believe is called TEGA) the mission scientists will look for that “fingerprint” and subtract it out, if it is there at all.