It depends on the situation. Can you safely restrain the mother or are you likely to get injured?
I tend to be very proactive (but my view is biased because I was paid to do something). Once the waters have ruptured, there is no harm in having a feel so long as you are relatively clean.
1) Put a halter on and tie the mother (make sure there is enough slack so that she can still lie down)
2) Wash the rear end with a mild soap and clean water. And tie the tail.
3) Remove your hand/arm jewelry. Wash your arms and hands (and you can also wear rectal gloves)
4) Have a feel inside:
A) Do you have a head and two feet? Yes.
i) Is the calf moving? Yes, you can step back and observe her (I tend to agree with Dun's estimates)
ii) Calf is not moving. Pinch a foot or tongue, if there is a response, you can step back and observe.
iii) No response after pinching. The mother will probably need help. If you don't know what to do call for help (vet).
B) You do not have two feet and a head: The mother will most likely need help.
i) If you do not know what to do, call for help
ii) If you know what to do, know your limits.
Remember that the faster you call for help, the better the chance of getting a live calf. Vets are expensive, yes, but once the calf dies, the vet cannot bring them back to life.