Will a compression test tell me exactly what the issue is?

I'd say its a wise first step before you start tearing things apart. 

It will help in diagnosing the problem. But with all things you need to eliminate the obvious 1st when fault finding. Worn piston rings can show as low compression so it is worth doing a test considering it isn't difficult if you have the tool. 

6 hours ago, shreksfocus said:

Will a compression test tell me exactly what the issue is?

NO!

And don't believe the first garage who categorically tells you the compression test isolated the problem!

The old oil in the cylinders trick does not always work.

It is a guide, and a useful guide, but it is fallible.

I had a car with a warm starting problem, one garage said definitely rings, another said maybe valves. I had to remove the head myself, as I was not convinced, and there was a gaping hole under one of the valves. That was an old 6 cylinder Triumph 2000.

Then years later, on a Sierra with the CVH engine, which was burning loads of oil. When I bought it I noted the dipstick on min and the half empty 5l tin of oil in the boot, bit of a give-away, but it was quite cheap, so I took it anyway! Again a couple of compression tests gave varying results, apart from compression being bad.

I had the valve seals done, which was a quite cheap, easy job on the CVH, The garage mechanic shook his head, saying he thought there was a deeper problem. To start with, it seemed he was right, the oil consumption was much better, bit still high. However over about 9 months, the consumption dropped to almost nothing, and I had over 6 years of really good use out of that car. I suspect the rings slowly un-gummed once the oil ingress had stopped.

Before committing to any major expensive job, I recommend you get at least a couple of expert opinions.

However, a compression test will tell you if there is a compression problem, and I think you then have two choices: Replace the engine/car, or remove the head to confirm the source of the problem. There are not many engines where the seals can be replaced with the CH on, like the CVH engine.