Listenings the key....and that relates to what spin said. Heres what i always do: Open the joint that im bout to sample, and then scan through the part im bout to sample....Aftwards, i go oever a few more times and then i know i want whatever part it is....So now when u listen to that part again, dont watch a clock or nothin like that, instead count in "4's"....Count in 4s four times obviously for a 4-bar loop. If the sample feels like it simply isnt in time with a "4-count" then it aint worth yer time basically. Sometimes theres obviously exceptions tho; like when all it would take is a small timestretch since u can hear it working, but its just a little off, in that case its all good and u just need to tweak it a bit, but for the most part if yer regular timing in yer head dont match up then it really aint a descent sample to let ride for a whole 4 bars. You can also chop the fucker up and play parts of it out like chords similar to what premo does, but still thats another subject that doesnt apply to what yer askin.
STEEZ
(oh i forgot to mention: the information i dropped above is in regaurds to a 4/4 meter.)....If the Meter were different, you would obviously not be counting in 4's everytime.)