engine repair

There are a few means for checking valves without pulling heads: a compression test, a leak-down test, inspection with a bore scope (may also be called an endoscopic test), you can remove valve covers and check valve stems (for proper angle, valve stem heights, lash, etc.) A scope can also check on condition of pistons. A leak down test can also check for ring blow by. Compression test can also indicate a bad head gasket. I'd start with the simpler tests like compression, if you see negative results do a leak down or get ahold of a bore scope. You also have variable valve timing on Tahoes (depending on year) if your timing feels off, it may be a non-working VVT solenoid.

Not necessarily unless you want to do a visual check. A vacuum gauge can tell you quite alot about your engines overall health and your valves, it wont pinpoint what cylinder your having an issue on but for that you can check the health of your valves per cylinder by doing a leak down test. I built my own tester pretty cheaply with things i had and a couple things i needed to round up to complete it. The actual numbers youll see dont mean anything its the variance that tells the story, 100psi is whats commonly used (at the air source) because its easy to turn it into a percentage of leakage. Any used motor is bound to have some minor leakage to begin with, you're looking for large differences from one cylinder to the next or large percentages of leakage across the board to point you in the right direction, similar to doing a cylinder compression test.

I suppose you might be able to see them if you use a bore scope down the plug holes or into the intake manifold or into the exhaust ports.