1. Sniff for gasoline at the intake after the engine has been turned
off
for a minute.

2. Look at the injector signal pulse width while the engine runs.
Leaky
injectors will cause the pulses to be narrower than usual because the
computer will try to compensate for excessive fuel delivery by opening
the injectors less.  Any good mechanic can read the computer output
and will know when the pulses aren't right.

3. Hook up a fuel pressure guage and see how fast the pressure drops
as the car is parked.

4.  Pull the injectors, reconnect them to the fuel rail, and look for
drips
while the system is pressurized.

Don't try #4 except as a last resort, and I doubt you have a leaking
injector but more likely a problem with the pressure regulator or a
check valve.  If the problem is just gas fumes, check the charcoal
canister and its plumbing and valves.  Get a Ford or Mitchell manual
and follow its instructions before replacing parts.  For fuel-computer-
emissions systems you want these books and not Chilton/Haynes
crap.

What makes you suspect that the injectors are leaking down when
sitting?

The abnormally high idle you state is not the result of a leaky injector
causing the engine to burn off the excess fuel leaked into the engine.
The engine would burn all that fuel at start-up, and possibly give you
a long-start at start-up. [basically, flooded]. Also, an injector with
a worn or open pintle will bleed pressure out of the immediate fuel
line. Again, a delay in start-up.

A leaky injector won't spray as well as needed, and the resulting high
idle is PCM compensation. As stated above, a fuel-pressure check of the
injectors is a good place to start. Caveat> usually best left to pay a
shop for the test.