Whether a loop is in a minor or major key is determined by the tonal center of the loop. It CANNOT be both at the same time. However like deathy says below it can work for both depending on the circumstances of the loop.
Is it necessarily either? If it only uses the 1st (F), 2nd (G) and the 5th (C), then it works for both. In my sample collection, most of the time, when there's no M or m, then it has been both.
Let me clarify a bit on what deathy is trying to say here.
The notes in F Major are F, G, A, B♭, C, D, and E
The notes in F Minor are F, G, A♭, B♭, C, D♭, and E♭.
Notice how all the notes in these two keys are different aside from the 1st (F), 2nd (G) and the 5th (C)?
If the loop uses these three notes then the loop can POTENTIALLY be used in either of these keys. However it CANNOT be in both at the same time. To tell whether a loop is in F major or F minor listen to the notes. Listen to the chord progression. Does it sound like a F minor chord is the home of the loop? If thats the case then the loop is in F minor. If it sounds like an F Major chord is the home then the same rules apply. The most important thing is to listen to the cadence (resolution) of the bass.