Sorry.  Never know where to start.

Better to put a #3 Phillips screwdriver with an appropriate insulating
plastic handle into the connector in the boot and hold or prop it within
1/4" of a ground while someone else cranks the engine.  Don't just look.
 Listen, too.  In daylight it can be hard to see the spark but you'll
hear the hard "snap" clearly even if you can't see the spark.  Needless
to say, secondary ignition voltage is high, so be sure you don't make
yourself the ground return path for the spark.

If you have fuel but no spark the plugs will be at least slightly wet
with gas.  Pull one and see.

You could also check for voltage across an injector (assuming the engine
runs smoothly when it does start we should be able to assume good
injectors).  You should see the needle twitch if you use an analog
meter.  That will prove the ground return has been connected.  (As I
recall, the ground goes through the MR, but check the wiring diagram.)
Next, put a folded rag over the fuel filter and loosen the banjo bolt.
Fuel will come out if the system is pressurized.  It will be apparent if
the fuel pressure is appropriately high, even through the rag.  Tighten,
turn on the key (but don't start) and repeat to make sure the fuel pump
is running.  (Guess what else is controlled by the MR?)  Among other
things, gasoline is bad for you if swallowed, bad for your eyes, and
inflammable.  Follow appropriate safety precautions!

Without looking at the wiring diagram, I recall that there are at least
a few symptoms that track back to the main relay.  Because main relay
problems can be very intermittent they can also take a long time to
diagnose the "right way."  The first one I found took nearly 2 months
(it failed once or twice a week and the car usually started on the 4th
or 5th try) and I tracked it back with a meter step-by-step so I *knew*
what it was.  If your eyes are good enough (mine no longer are without a
magnifier), you can actually *see* the greyish ring around the middle of
each joint that has gone cold from repeated thermal cycling.

The next time I encountered the same symptom on another Honda I just
pulled the main relay, inspected it, and reflowed the solder joints.
Problem solved...at least, for the past three years.

This is not to say that your problem *is* the main relay.  I work with
someone who also owns a '91 (as I do) and his has yet to fail while I
have had the failure on two of our Hondas (the other was a '90 Civic).
There are any number of other things that could go intermittent and
cause failure to start.  Connectors, igniter, fuel pump, etc., are all
possible.

But it's quick and nearly free to reflow the MR joints if you already
own or can borrow a soldering iron and know how to use it.  Just be sure
to flow some fresh solder into each joint so the connections are good.

Hope this helps.

The fuel filter will virtually always cause symptoms at high-RPM,
wide-open throttle *far* before it can cause a starting problem.  My
money's on the main relay.