A couple of relatively easy ways to check if it's the relay:

1. When it doesn't start, check to see if the fuel pump is energized when you turn the key to the "run" position. The pump should buzz lightly for about a second, then shut off. If it doesn't, it's not getting power.

2. If the fuel pump isn't being energized, remove the relay and bridge (I think, you may want to search the forums to check) pins 6 and 7 with a jumper and see if that turns on the pump. If so, then the relay (at least) is faulty. Note that this pressurizes the fuel system, in case you are doing any other work.

In my case, I had similar symptoms - randomly would crank, but not start - and the relay turned out to be the culprit.