Code to tell if a user is a supporter

Is there any php code that I could use to tell if a user is a supporter or not?

I am thinking of something along the lines of:

if is supporter then

else
show an ad
endif

I'd like to be able to add things to a theme, such as sidebar ads to a theme, but if the user is a supporter, then the sidebar ads wouldn't show up. Or going the other way, add things to the sidebar for supporters, which will not appear if the user isn't a supporter.