You can sense engine revs with the AC ripple on the alternator output (
https://www.electro-tech-online.com/custompdfs/2008/10/H-SEN-KMT-RPM-D100-011US.pdf)
However, the signal is very small and it may not be present at cranking speeds. Alternators have a rotor winding, which is fed through the slip rings. That controls the alternator power and is used to regulate the voltage. The current for the rotor comes from the alternator itself, so it won't ever generate anything unless some other rotor current is provided. That comes from the ignition warning light.
When the alternator starts to work, it generates enough voltage to provide its own rotor current and the ignition light goes out. Until that happens, you won't be getting any conduction of the main diodes that feed the battery, so there won't be any ripple. That was why I suggested looking at the alternator voltage before the diodes, where there will be at least a volt of AC even at cranking speeds.
Watch the ignition warning light as you start the engine. On some cars, that doesn't turn off until the engine speed has gone a little over tickover, but I am fairly sure that it won't go out at cranking speeds.
If the ignition warning light does go out as the engine starts, you could use that but be careful. If the ignition warning light bulb fails, there is no rotor current so the alternator never generates. I would also suggest a timeout on how long you run the starter motor.
Alternators are not difficult to take apart enough to get to the diodes. The diodes are huge and on large heatsinks, and the wires are about 1mm in diameter so there is nothing little or delicate.