I think it is hard to hear....by ear the headspeed on the Raptor 50 or 60 because of the gearing. A better option for now would be to try a tach. I have used the VD0 tach in the past with good results. They are cheap but a bit of a pain to set up right in the heli. With a little time and effort it will do the trick. The other more expensive option is to get a GV-1 and you won't have to worry about your headspeed ever again!

Here is a link to the VD0 mod! http://www.rcheliweb.co.uk/

Go to hints and tips and then look for DIY tach