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.
It depends on the conference. In recent years, the College Art Association, an annual conference for art historians and artists, has instituted a ["Pay-What-You-Wish"](https://www.collegeart.org/news/2019/02/05/pay-as-you-wish-is-back-for-caa-2019/) option. This was instituted after many adjuncts, students, and others expressed their inability to pay fees to attend an integral conference in their field. I mention this in case you are a member of an organization that holds conferences. As a member, you may be able to make your voice heard through elections and other ways that can foster a greater awareness of the high costs of attending conferences for underfunded faculty members. Hope this helps.