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 can't answer how to find conferences as this will depend heavily on the field (or even subfield) that you are in. However, in many fields there are websites/mailing-lists where conferences are announced. 

As for finding conferences that will allow you to attend for free. Your starting point will always be the conference announcements. There are several options:

* Some conferences/meetings will not charge registration fees at all. These are usually smaller meetings (< 100 attendees).
* Some conference organisers may be willing the waive a fee if you ask nicely. (You are asking for a favor, so be nice!). Some points to consider if doing this.

	+ Smaller conferences are more likely to waive your fee. For big conferences, (part of) the organisation is often out sourced to a commercial company, making a waived fee less likely.
	+ For the same reason, look for conferences that are hosted by a university or research center, rather than at a commercial conference venue. (Again organisers are then more likely to have the flexibility to waive a fee.)
	+ Conferences organisers are significantly more likely to waive a fee if they feel your presence will be an enhancement to the meeting. It helps, for example, if they have heard of your work before and it fits with the goals of the meeting. So look for conferences organised by people that you have interacted with scientifically. At the very least, have a plausible reason for wanting to attend this particular conference.
	+ Look for conferences that are somewhat local. Flying half-way across the globe for a conference does not lend your appeal for a waived fee much credence. In particular, avoid conference that would require you apply for a travel visa. (There are just too many people for Visa invitation letters for conferences.)
I’ve never seen an official list of such conferences: who would register if it were advertised you can go for free? In addition going to a conference for the actual talks kinda misses the point: a conference is not an obligation to go to every talk, but an occasion to meet people so short of avoiding contact, the first thing people will ask is who are you, your affiliation etc and will rapidly discover you are not registered.

It is not pleasant as an organizer to see people crashing the conference: there is a real cost to organizing an event and common courtesy would be to ask the organizers for a discount or a free registration: the worse possible scenario for you is to travel somewhere and be blacklisted from the event after getting there and arranging accommodation.