One of the questions recently made reference to an exhaust failing the "dollar bill" test.
How does one perform it and what does it tell you?
The dollar bill test is quite simply using a dollar bill (any denomination ... doesn't have to be American currency, even) and placing it against the exhaust tip while the vehicle is idling. You are looking/listening to what the bill does while against the exhaust tip. If it just flutters there without making much noise, you have healthy exhaust valves. If it rattles against the tail pipe, you may have a burnt exhaust valve (no guarantees here), which would be a basis for further examination, such as compression and leakdown tests.
What is happening is this, when an exhaust valve is burnt, it is no longer sealing against the valve seat. When idling, there is usually a great deal of vacuum created during the intake cycle. If the valve is burnt and leaking, you'll get reversion. Reversion is where the exhaust flow changes direction back toward the cylinder, which should not be happening. By placing the bill on the exhaust tip, the bill will rattle. The noise will be very noticeable.
Please note, in order for this test to work correctly, you need to have a good exhaust system (without leaks). The bill also has to have a good fit over the exhaust tip. If it's too small, the effect wont occur. I don't know how well it will work with exhausts which have cross-over or h-pipes built into the system. The engine has to be idling as the vacuum is much less and the exhaust flow is much greater when the engine is off-idle.
Being in the UK and having no access to dollar bills, I assume that it involves holding a dollar bill over the end of the tail pipe of a running car and looking for signs of oil on the note.
This would show up if there were wear to oil seals within the engine, such as valve stem seals. A similar test in the UK is to place a white cotton rag across the tail pipe and see if it gets excessively blackened by emissions.