I said knock, but what I actually do is a combo of the two if I'm in a situation where I don't think my knock will be heard and/or I won't be able to hear a reply:

1. Try knob slowly; if locked, stop.
2. If unlocked, open the door a crack and knock at the same time, so that the person will hear me and I will hear them.
3. If no reply, open door slowly until I'm sure no one's there. ;)