Originally Posted by SloEvo12

Go to Lowe's/Home Depot or whatever is most convenient and get a plastic PVC cap. Usually if you have approx 2.5" inlet then a 2" cap, usually if you have a 3" inlet then maybe a 2.5" cap. They're sized differently than they are labelled. Anyhow, get a PVC cap that will fit into a silicone coupler on your turbo (that's where you'll be pressure testing from). Then go to Advance Auto or Pep Boys or whatever and get the heavy duty valve stems - they're the metal ones that actually have a nut to tighten them down (as opposed to the ones that are all rubber and press/pressure fitted/held). The heavy duty ones are nice because they require a less accurate drilling measurement and tend to seem easier to fit into PVC. After you've gotten the appropriate cap and valve stem you obviously assemble the stem into the cap and then clamp it into your silicone coupler. I'd generally suggest to use ~20psi regulated from an air compressor. It WILL leak through because you're going to have open valves and such, but you can still have enough time to hear things. Use soapy water if there's a spot you think is leaking, but aren't sure. You can also use carb cleaner and spray it at a suspected leak while the car is running and see if it effects idle. I think that covers a lot of it...