Thanks to all for your replies!
A little more info. The generator is a small 4KW, 7HP, gasoline engine. It does not have a tach output. It does have a low oil sensor but not an oil pressure sensor.
Trying to keep this as simple as possible, here are my thoughts on your suggestions.
1) No tach output requires some type of sensor.
2) Crank current requires a sensor, or maybe read battery voltage?
3) No existing oil pressure sensor. Retrofit probably not an option?
4) As far as I can tell, there is no knock/detonation sensor.
5) Trying to measure a tacho signal from the charging coil for the starting battery may be possible with no special sensors?
6) Measure AC output frequency. Once the alternator is producing ~35+Hz, stop cranking. Requires measuring Hi Voltage frequency.
7) 120 Vac relay attached to generator output.
It seems the simplest first attempt would be to use a relay with a 120 Vac coil.
a) Begin cranking.
b) Monitor state of relay.
c) If relay closes, continue cranking for a short time then stop cranking.
d) If relay does not close and it has been cranking for 5 seconds, stop cranking, wait for 10 seconds, try again, goto a)
e) Check state of relay and make sure it remains closed, i.e. generator is running and 120 Vac is being output.
Of course this process will require some experimentation. The first thing being to attach the relay coil to the generator 120 Vac output and see when it closes. Does it close while cranking but the engine hasn't started? That would be a problem. I'll report back with my findings.
Thanks again for your suggestions.
Bob