How can i tell if i have an allergic reaction to lidocaine?

Hard to tell: Allergic reactions to Lidocaine are very rare. Most "allergies" with injected Lidocaine are caused by the adrenaline/epinephrine that is included with the lidocaine, and is felt as rapid heart rate and anxiety. A true allergic reaction is more serious, and could include swelling of the lips and tongue, asthma/breathing trouble, nausea/vomiting, low blood pressure, and fainting.

Possible: The old ester based local anesthetics like novocain (procaine) did cause allergy in some patients. The newer amide agents not so much. This would include lidocaine, bupivicaine, ropivicaine. Hives would be he most likely symptom and much more rarely anaphylaxis which is low blood pressure and swelling if the airway.