I assume that you'd closed the seacock disconnected the discharge line from the head and seen if any CLEAN water comes out of the toilet if you try to clycle it. It should be a pretty good pressure. If it doesn't do that, then it's the toilet. I don't know if there's a joker (or other check flapper/valve at the discharge which might be reversed?)

The the next thing to do is to see if you can pump water out of the discharge hose (w/out the toilet). If you had an adapter, you could try the garden hose method. You want to check for a blockage, or a bad y-valve, or ...