IN/OUT pin of the crystal circuits
You should measure the waveform from the OSC_OUT pin as that point is driven by the amplifier. Then adding some extra loading on that pin (the loading of the probe you will use to measure the voltage) is not an issue.

If you measure at OSC_IN you could be unlucky and disturb the circuit so much that it stops oscillating !

The internal circuit should and will do the same as you should so use OSC_OUT. That pin will not be used directly though there will always be additional buffer circuits present to feed the signal to all other circuits.