You could measure resistances, but IMO to be sure it works you really need a 12v supply to "bias" it: +12v to red, -12v to black. Then, manually rotate the metal rotor and you should get 0 to 0.7v for the lower voltage and about 5v for the higher voltage measured blue to black.
Dist. removal:
1. With engine out of car undo the two bolts that clamp dist. to head. Easy.
2. In situ. remove dist. cap first (3 small hexagon bolts - use 1/4 inch drive socket and/or ring spanner (wrench). Awkward and slow but you'll get there.
3. Rotor arm is tight - ease it off by "jiggling" and pulling - no screw.
4. Be careful not to damage hall sensor connector/wiring. A metal clip needs removing from the connector before it separates.

The first time I removed a 740 dist. took me well over half an hour. Now it's maybe 10 minutes. If you can get a cheap one you could maybe take a chance it works and test at home - many high mileage 740's will have replacement hall sensors / distributors anyway. You can probably see if this is the case.
Good luck.
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BillB