You can look at the tapeheads. If there is a gap between the two track elements of apprx the
same size as the elements themselves, you have a 4-track.
If the elements are close together, covering almost the entire tape width, it's a 2-track.
A tape recorded on a mono machine will typically have one track playing fine with the other track
playing something else in reverse.
On a stereo machine, you must then play only one channel at a time, typically the left one.
Having said that, there were uncommon recording formats around that will only play correct on the same type of
machine that recorded the tape.
Martin