The easiest two ways...First and cheapest carb cleaning/brake cleaner while it's running spray around the intake normally it will idle up when you find the leak. Second and most effective is a smoke machine things are amazing.
Find a repair shop near you with a smoke machine, seal any exit areas, and pump it through the valve cover until you see smoke.
The stuff dissipates almost immediately, and won't effect anything. Some times it takes a while for the smoke to fill an area and find a way to escape, but there really isn't much better of a way to find if there is a leak easier than that