I always make sure the plate is flat, they can get raised up between the bolts so they would leak right away, and add a thin coat of "right stuff" on either side of the gasket and make sure the plate and intake surface are totally oil free and it will not leak again. To check if it is blow you can sometimes see a big puddle of oil in the plenum looking down the throttle body, sometimes you can even see blown out sections of the gasket in there or the last test I do is pull the PCV out of the valve cover, plug the hole in the VC then put the palm of my hand over the oil cap hole with it idling and see if it starts building vacuume and trying to suck in my hand, if there is vacuume there then the gasket is leaking again.