Ideally you should start out by timing the source material to accurately determine it's BPM, You can use tap-tempo or a stopwatch and you want to avoid trying to determine the BPM or length by nudging loop points around because you are essentially setting your own point of reference instead of referencing the unrestrained source material in a way which is actually more intuitive and natural.
When you start off on the right foot everything becomes so much easier, I mean once you understand the relationship between BPM and loop length it opens a door to better sampling.....you should be able to look at the length of your sample and figure out the BPM if you cut it right, but if you are just hacking away your best bet will be to use tap-tempo because you don't have any clue where you're at.