Find out by asking your charge nurse or unit educator (or whomever is the best resource for education on your unit) if it is common practice for nurses to perform that skill on your unit.
If it ISN'T, page the doctor and let him/her know that you can't do it.
There really should be a policy if the hospital allows you do perform the procedure. It is nursing policy that guides our practice. These policies have been developed from intense nursing research, and by using these policies, we are thereby using evidence-based practice.
What I mean, specifically, is that we are not trained by doctors, but rather by nurses and nursing research. So, just because a doctor shows you how something is done, does not mean you can automatically perform a procedure.
Perhaps find out how you can start the ball rolling on developing a policy for this skill. Take the initiative...nurses don't do it enough