Best way I know of is to keep flying... eventually you'll find some.

Slightly more seriously: I"ve found the "real world" wx info in FS9 to be even less reliable than real world wx info in the real world...

Some turbulence can be easily deduced by an understanding of the big picture (converging fronts, etc), but there are always surprises. And sometimes a lone PIREP describing turbulence turns out to be nothing to be concerned about.