When I used to play in a band, I quickly learned that carrying a decent cable tester made the difference between wondering whether the PA system would work — or whether it’d cut out in the middle of the second set and make me look like a complete ass.

This is the one I ended up carrying, mainly because it’s cheap and effective.  As you can see from the picture, it can handle balanced and unbalanced mic cables, 1/4″ and 1/8″ cables — or best yet, any combination thereof.  For example, if you’ve got a balanced 1/4″ to XLR, just plug each end into the appropriate hole and you’re good to go.