Hi Jan, We do an "is it salt?" test with our in-house x-ray equipment: move the detector in as close as possible, and swing the 2-theta arm out to 45 degrees; take a 10 degree wedge of 1 degree images, 10s or less. If you don't have the ability to swing the detector in 2 theta, you might want to collect a wider wedge. If it's a small molecule xtal you should see strong high res reflections on some of those images. If you don't see anything you can be pretty confident that it's not salt.
Our "is it protein?" test is very long exposures of 0.2 degree images in dezinger mode, 1x1 binning, with a 30 or 45 degree step between images, hoping to see any reflections close to the beamstop. Neither test is completely definitive, but generally work well for us. Good luck!
