If you want to do it in your head, some engineers use 10 seconds as a reference, but to get a GENERAL bpm, (usually to match delay times) you count the number of beats in 10 seconds, and then multiply that by 6 to get your bpm. If it doesn't match an exact number, then whatever Big L used was patched together in the sample and/or played live and not looped completely to an exact bpm.
But this way is really easy.
Count the beats (4 beats per bar) until you get to 10 seconds. Say you count 16 beats (4 bars) times 6 (to get 60 seconds) = 96bpm.
If it's 95 bpm, then you may need more than 10 seconds to count the beats. Say 12 seconds, and multiply it by 5 to get your bpm. (You'd count exactly 19 beats in 12 seconds for 95 bpm)
If you want to just say fk it and count the actual beats, you can do that too. Just literally count out loud, the beats until you get to 60 seconds. Most likely, it was recorded on an actual tempo. So if you find it, either ball park or right on, and adjust from there, you'll be okay.
That's how to do it in your head.
If it's only the acapella, then you have to imagine where the kicks and snares are. In pro tools, you can hit the down arrow while it's playing at the beginning of the beat, then the up arrow to end the selection of say... 4 bars.
Now you click on "identify beat" and it will tell you your bpm. But if you don't have it exactly lined up with kicks and snares, then it will give you ballpark. If you're close, you'll know it, because it may say something like "87.08", so you'll know it's most likely 87.
A way to do it to get a more accurate reading is to open the song in any wave editor program (cool edit, soundforge, etc) and select as close to where the kicks will land from the first one to the next one. If you know it's 1 beat, then you can look at how many miliseconds it is, and divide 60 by that. (e.g. 60/ 0.625 = 96bpm)
Hope this helped to do this in your head.