Possible wasp allergy. How do you know if only stung once?

Symptoms? It is important to identify the symptoms, whether they were simply local (redness, swelling at the sting site), or systemic (hives, anapylaxis). An allergist can test you to confirm this, and can tell you the chances of having a systemic rection in the future. If originally local, about 5% chance. If systemic, then about 60% chance.