A compression test is a good indicator if your pistons/rings are damaged of if your HG is lifting/Blown. There are other symptoms of the above but if you have too low of compression in a cylinder there's something wrong with or without the other symptoms.
Ill ad pictures when i take them,for now I'll just have to use text.
1) Get the engine to normal running temperature.
2) Remove the Spark plugs leads.
3) Remove the king (not the distributor end)
4) Remove Spark Plug 1 (doesn't matter which order)
5) Screw the Compression Tester into the spark plug hole.
6)While you are looking at the compression tester get someone to start turning the engine over via the key while holding there foot firmly on the accelerator all the way down.
7) Let the compression build until it stops and then stop the person from turning the engine over.
8) Write down the Compression results/Remove Compression Tester/Replace Plug
9)Repeat step 4 - 8
10) Compare results,they should all be within %10 of each other across the board.If not you have a problem and this could be the problem for the loss of power etc
Personally i do the test twice to be 100% sure.
Also to eliminate ringland failure,drop a bit of oil into the cylinder where the compression is low and redo the test,if compression goes back up its pointiong towards ringland failure.
Ill be editing this with what a 3S-GTE (TURBO) and 3S-GE (NA) good compression figures.
Link to Compression Tester I use - Compression Tester
*If anyone thinks there is more to add or I made a mistake just reply here