I know a number of Americans living in Canada that have successfully traveled to Cuba. The US has not fined anyone in a very long time over this. The US, however, could find out if it wanted to that you flew from Canada to Cuba. The flights pass over US airspace and the passenger manifests are shared with the US government. This means that if asked at some point, you should either take the 5th or answer truthfully.

I think if you're just going as a tourist, you have nothing to worry about. I presently have Nexus, so I have some trepidation about going to Cuba, as I'd rather not lose that.