How do you know if a needle has broken off in your arm if you did not inject yourself?

The simplest is an xray. Check with your local clinic and give them your story. If it is under the skin, you should be able to feel it. If in deep muscle tissue, an xray will be needed to locate it. Either way, you might need local site-surgery to hve it removed. If it was a contaminated needle, please don't wait. (THIS MEANS KNOWN TO BE ABSOLUTELY NEW AND STERILE.)