[beamcurrent]

Now I'm no expert on tape, I'm a camera person.

But it may be that if you "dust" the tape with a VERY fine iron or iron oxide you might be able to see the magnetic tracks and determine if it is quad, helical, linear audio by the direction of the tracks.

This technique was used for manually editing quad tapes and you had to cut it in the correct place!!

Attached is a picture of a Quad splice machine that I have in the museums collection. Unfortunately it is missing some parts, but you get the idea.