It looks like the traffic is really high today, probably because of the introduction of hats.
I was wondering, is there a way to check statistics about the traffic on math.SE, and on the other SE websites?
There are publicly visible statistics available on Quantcast. As SE uses the Quantcast tracker, they don't have to guess and this is actual traffic data.
You'll have to wait a bit though, the graphs are only updated later the next day.
You can take a look at the Alexa rankings for any site, but those aren't very good. Stackexchange keeps its own, more detailed data on traffic and usage, but this is not available to regular users.