JUST READ THE HELP! (Actually, doc det.) In there, you will see this statement:
det computes the determinant from the triangular factors obtained by Gaussian elimination with the lu function.
[L,U] = lu(X)
s = det(L) % This is always +1 or -1
det(X) = s*prod(diag(U))
Seriously, almost always just reading the docs for a function will give you at least some clue as to how they are written. (Not always.) But at least they will usually give you a reference, and for basic codes, they will give you a good idea of how they are written.
Sometimes, you can even look at the code itself. In that case, do this:
type det
Here, it will tell you only that det is a compiled code, but at least you can look.
