OK, the design is different between the 5.7 and 6.1? I think I will pull that manifold and look anyway, I am no sure this engine is original to the car so maybe the manifold has been bodged onto the wrong kind of motor or something.
edit: for a smoke test, is it enough to block off the PCV and air balance tube and see if the smoke is getting into the manifold? If the smoke is getting past the rings or valve guides or whatever what could I do to narrow it down other than maybe pulling a few spark plugs?
What year is the vehicle? Can you see any evidence that the engine has been tampered with or possibly removed? What does the odometer say for mileage? Does it look like it has been properly cared for or does it appear to be neglected? Have you checked for Diagnostic Trouble Codes?
The design between the 5.7 and 6.1 are basically the same, but the intake manifolds and gaskets are considerably different....5.7 is plastic with o-ring type gaskets and the 6.1 is an aluminum manifold with real crappy aluminum gaskets that have lousy rubber seals surrounding the ports. See the pix below of when I took my 6.1 apart. Most of the rubber seals had been ingested through the engine, and the others were all distorted and half gone. The engine did have about 165K on it and probably never been touched.
Unless someone has grossly neglected the engine over a long period of time, I doubt you'll have a problem with the bottom end, the pistons/rings. If it has over about 100K you'll probably have some bad valve guides. If it's an 05-06 or early 07, dropped valve seats can be a problem, especially if the engine has been overheated.
The things you can do to determine the basic condition of the engine and find where the leak is coming from is to do a compression check first then, perhaps a cylinder leak down test if the compression is not real good and, finally, a smoke test if you're unable to find anything. You could also start with a smoke test if you wanted, but I wouldn't. Usually, a compression check and leak down test will tell you everything you need to know though.
FYI, 5.7's have an EGR Valve on the front passenger side head, as said, a black plastic intake with the PC Valve on top rear of the passenger side. 6.1's have the aluminum intake, no EGR Valve and the PC Valve is located on the front passenger side.
Smoke testing an engine is kind of redundant, a PITA and time consuming because you'll need to isolate each cylinder and check them one at a time with both valves "supposedly" closed....top dead center on the compression stroke. This is better done with the cylinder leak down test. IMO, the smoke test is best for checking for intake/exhaust leaks and such.
On a 5.7, the PC System and, especially the PC Valve, are one of the biggest weak spots on a 5.7. It's difficult to determine if the valve is good or bad, because the spring in it is so weak to begin with. After time it gets weaker and the old method of "shaking the valve" is not an accurate way of checking them. I've never seen one that didn't shake, no matter its condition. Just change it every 15-30K and be done with it. You'll be able to see how bad the PC System is by looking at the back side of the throttle body butterfly. It'll be all caked up with crud and, worst case, there will be oil puddling in the intake manifold plenum.