Doing a test through your provider (if they have one like dialing 933) is the safest way absent dialing 911.
If, however, you want to brave the call to 911, don't hang up when the 911 operator picks up. Just tell him/her that you accidentally dialed 911 as a preset speed dial and didn't want to hang up without telling him/her that it was an accident.
Without telling the 911 operator that the call wasn't intentional, they'll likely try to call you back to see if you had a problem. And...if they can't get a hold of you (like me one day when I was testing my PBX), they could send a patrol car to check up on you. The reason for this is because they don't know if you had a medical issue (or criminal issue) that left you unconscious after dialing 911.