how to find conferences that allow sit-in's
I have to say that I can't remember ever having my badge checked at any conference. In fact, by the second day, i've often lost my badge. What does sometimes happen is having your badge checked to get onto the campus in the first place. Also, people often have greeters at the door during registration. 

I've "sat in" on plenty conferences before when they have been happening on my campus anyway. I'm not sure I'd risk travelling to one on that basis though.