What key is the piece in? And is the chord written as B b5 (that is, a B with a flat 5th) or is it written as Bb 5 (that is, a B flat open 5 chord -no 3rd). The key will make that usually obvious.

When in doubt, why not take a look at the actual notes in the sheet music? - it doesn't take too long to figure them out, even if you are not a fluent reader.