Question: For faculty lack of funding, how to find conferences that allow people to sit in without paying? For those that don't allow officially, will they check attendees' badge?

Answer: 
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.
Association conferences sometimes have scholarships or low cost opens as others have mentioned. Also, there may be sponsoring organisations for the conference that you might be able to approach. Otherwise, sometimes offering to volunteer or help organize is an option as well. 

"Sitting in" is very against the culture and intention of the conference. Conferences though expensive are usually run as a lost. Venue hire, staff, etc are all very expensive and basically that is what all the money goes to. Most of the presenters are likely to self-fund their travel except the famous keynotes, etc.