Why don't you have a sliding scale of options to state how dominant or submissive someone is, alongside the option of stating that they might switch between the two?

➜ Very dominant
➜ Fairly dominant
➜ Neither dominant nor submissive
➜ Fairly submissive
➜ Very submissive
➜ Switch - can be either dominant or submissive
➜ none of your business
➜ Don't know