Bergia are easy to test for. When they eat Aiptasia they keep the nematocyst intact and they collect in their gills. If you're not allergic, touch one of the gills. If it's a Bergia the gill will stick to your finger and will be torn off the Bergia when the nematocysts fire.
If the gills don't stick to your finger then it's something else. Wash you hands well with hot soapy water because most Nudibranch store the toxins of the coral they eat in their gills.