Kind of like checking a PCV valve by shaking it.  If it rattles, it's
broken and should be replaced.  If it doesn't rattle, it's gummed shut
and should be replaced.

Seriously, there's not a lot to go wrong in an ignition system.  If
you've check your points, condensor, coil wire, primary and secondary
coil windings, and the ballast resistor, and all of your connections
are clean and tight, you've pretty much isolated the problem to the
ICU anyway.

For $200 and about a 1/2-hour of installation work, you can ditch the
ICU entirely and get a modern capacitive-discharge multi-spark
ignition controller and a high-voltage coil, and be done with it.  You
would still be using the stock points and condenser, but you could
remove the ICU entirely and bypass the ballast resistor, so that's two
less parts to worry about.

Come to think of it, my '72 250C *might* still have the ICU under the
battery.  The previous owner replaced the stock distributor with a
Mallory Unilite, and I just added an MSD ignition box and coil.  I
don't know if the original ICU is still there, and if it's there, I
don't know if it works, but you could have it for the shipping cost.

I'll check this afternoon and get back to you.