Can the knock test tell you whether a stud is wood or metal?
I suppose it's possible although I've never noticed it myself. Although now that I think about it, every time I've worked with steel its been in an industrial setting (hospitals, etc.) which means it was covered with sound mat. 

If you want to know for sure, take rare a earth magnet and run it across the area. If it doesn't stick, well non evidence is not evidence as they say. But if it does, you know it's not wood:) Go horizontal till you get a hit (if you get one) then go up and down a bit to make sure you're not just hitting a screw.