[maicro]

Another literal answer to this question - spark testing. Take a sample to a grinder/belt sander and observe the sparks coming off - fairly crude, but you should be able to tell the difference between aluminum (no sparks), steel (mostly orange-ish) and titanium (white)[0]. That's really only enough to tell you the general material type though - the alloy and temper are also extremely important, as others in this comment chain have said.

[0] - https://youtu.be/GnSBSKTC834?t=504 - not super happy with this video for a quick overview to provide to people, but this timestamp does cover this specific discussion; if I find a different video that covers the differences more broadly, I'll link it here.