How to check cylinder compression:
How to test car engine compression
Basically it is a hose with a gage that screws into a spark plug hole of one of the cylinders. Crank the engine a few times without the distributor or fuel and check the pressure. You do this for each cylinder. Don't quote me on the specs, but I believe over 200psi will be just fine, and as low as 180psi would be good too. If there is a big mismatch, or if one or more are much less than around 150psi or so, then there could be issues such as the common exhaust valve being burnt, especially in #2 cylinder. Again, don't quote me. I one of the cylinders are way low, then this is the problem with not being able to get fuel economy.
You might be able to make or borrow a gage. Of course you could buy one also. Most repair shops should have one handy. Over here in the US, we can rent one from an auto parts store.
Hope this helps.