I know this is old question but its top search on google. So to answer the question directly, a GetFlags(); on the actor will reveal the most reliable flag being RF_WasLoaded for editor spawned actor. But only IF the actor was spawned in from a saved map. It is possible to have NO RF_WasLoaded flag if; you drag actor into level, and hit play.
However if you drag the actor into level, SAVE, then hit play, the actor will receive RF_WasLoaded flag everytime!
You can then do a simple if(HasAnyFlags(RF_WasLoaded)){ //im an editor actor! } like so.