I am studying meter and time signatures and was hoping someone could clear something up for me.

My question is : How does one know how many beats there are in a measure?

When I began studying this I thought I understood, but then I read an example that confused me.

In the example it say's that 12/8 is defined as compound quadruple, in that it has four beats and each beat can be devided into three parts. But how do you determine that since it could also (to my incomplete knowledge) be three beats with four parts each, or six beats with two parts each?

Any help would be greatly appreciated.
I think that's up to you to decide as long as you keep to the 12 beat period.
There are conventions to do with compound time signatures, that are easily learnt but also confused.

Such as: 6/4 is two dotted half-notes/minims in a bar/measure. Whereas, 3/4 is 3 quarter-notes/crotchets in a bar and is counted as simple time.

The three beats with four parts in might be 3/2, with the "parts" as quavers making up a halfnote.

From there its either compound, simple or weird stuff like 7/4 or whatever. Also, in a piece of music it is very easy to discern the beat if it has one, so just stick with common sense; with a bit of musical stuff as well.
Quote by Robbie n strat
In the changing rooms we'd all jump around so our dicks and balls bounced all over the place, which we found hilarious.

Little children should be felt, not heard.
a time signature doesn't always give you enough information to determine the beats in a bar

for example 9/8 can be legitimately subdivided several ways... it can be three beats each subdivided into 3... or it can be played lke a 4/4 bar with an extra eighth note tacked onto the end... or it can be something even more exotic like 2+3+2+2

time sigs don't give you enough info... like coffeeguy says, tap along or look how the notes are grouped
listen to dream theater!

time signatures are just ways of writing down something you hear its perception

4/4 is the same as 2 bars of 2/4 right?

anyway there are 2 types of time signature, simple and compound

simple are based on 2s compound on 3s

so 4/4 has 8 8th notes, and 3/4 has 6 8th notes....time signatures with a 4 are simple,

if you write 6/8 it too has 6 8th notes (like 3/4) but we dont call 6/8 3/4 because 6/8 is compound, its counted as two triplets

1 and a 2 and a, 1 and a 2 and a (think metalicas nothing else matters)

but 3 is counted

1 and 2 and 3 and, 1 and 2 and 3 and

both 6 8th notes... but 6/8 is closer to 2/4, as both have 2 beats in the pulse

so 12/8 is 4 triplets

1 and a 2 and a 3 and a 4 and a.

next experiment with real time signatures like 5/8 6/16 7/8 etc etc

everything i explained is all perception, a bar of 12/8 can be written in a bar of 4/4 by using triplets, you cant do this with 7/8....listen to erotomania by dream theater and read the score to it, learn to count through it coz this changes time signatures ALOT