right click on the chief engineer (left most officer panel) to get a good look at the rpm. or go into the conning tower (up through the hatch from the control room, the little cupboard where the attack periscope is) for an even better look at the rpm.
you can 'set' the rpm by simply ordering a certain speed, that is done from the speed dial in the bottom right of your screen. 1 knot or slightly below one knot will ensure 50 rpm. the chief will say something like 'yes sir, new speed 0 knots) but if you look closely he will actually make the speed 0.9 knots or something. thats enough to creep along while remaining quite silent.
to be the most silent you could possibly be, you will need:
-silent running
-engines completely off (all stop)
-decent depth (depending on your boat and the year of the war, i'd say anything below 160 or 180 is not bad)
that way, they can't really hear you. they can still ping you though, and they probably WILL because they want to find you, of course
so in addition to all of that, it helps to keep the boat pointed directly towards or directly away from the destroyer which is pinging (not easy, will come with experience)
many people will use many different ways to escape the escorts. my personal style:
as they come in for a depthcharge run (if you are convinced they actually have you nailed down precisely), take a run when the charges drop, the DD is above you and can't hear you anymore. run and turn away, just for a couple seconds, cut engines again and try to end up with your nose or tail towards the DD. now they have to find you all over again
this can go on for hours. if you use time compression they will have more success finding you, best way to escape is sit through it in real time
phew that was long, now i'm sure the other kaleuns have more to add to this