Ohh, I've done this. I bought some titanium bike parts and I was suspicious if they were titanium. I measured the weight of the bolts then dropped them in a graduated cylinder to get the volume, mass divas by volume is density, I then looked up the density and it was the same.
2. Easiest, most accessible testing method is scratching it on tile or glass. When scratched against glass (or ceramic tile), steel will probably leave a real scratch, aluminum will do nothing, titanium will leave a pencil-like line.
I am guessing that it was real titanium, just a different grade/alloy/treating process being passed off as something it was not or it's possible it's even the same quality/grade, just of unknown provenance (fell off the back of a truck) and its documents were forged. Seems kinda likely as Boeing says (as I understand from the article) they have tested the parts and it's the correct grade of titanium.
You are correct that this is what the article says - testing suggests it is in fact titanium, just maybe not the right treatment.
That would be harder, but one would think that a company making airframes for aviation, in a highly regulated environment/etc, would occasionally send off samples to double check them.
Getting titanium analyzed to a degree you could tell whether it is the right grade/alloy is cheap and fast - I can get it done for <$100 per sample.
Given the cost of what they are producing, how few they produce, and how much they sell them for, and how quickly you can get this kind of thing done, they could test every single lot of titanium they get and neither increase cost, nor slow down production.
This also isn't a case where there are lots of people in the middle - this supplier is the ones machining and producing the final product from titanium alloys.
Also, if you change suppliers, wouldn't you at least test the stuff they give you the first time?
Except the article says they only tested it after they found corrosion reported back to them (IE they did not discover or test it ahead of time), and that testing they have now done says it is not treated properly.
So it doesn't appear Spirit has sufficient testing, or that the titanium passes all the tests.
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.