Even though you've let it slip, I don't think it would hurt to ask them what they think about polyamory in general. If they react favourably, then let them know explicitly that you are polyamorous. In the process, it will undoubtedly come out whether they are poly or mono. There's also the possibility that they've never really thought about it, and are just following mono rules by convention.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker