As the title says for private airports and planes that fly in and out of them is there a frequency that they use and if so where can it be found?
Referencing AIM: Services Available to Pilots/Summary of Recommended Communication Procedures (Link).
No Tower, FSS, or UNICOM -Self‐announce on MULTICOM frequency 122.9
However - as you may expect, the short of it is - it's private - so nobody will be listening most likely. I'm only listing the AIM reference as the most authoritative source - which even then - likely doesn't even fully cover this.
I'd have to imagine that some of these are owned by individuals living on a ranch like setting in a remote area, and assume that they are going to be the only individuals using the runway (unless an emergency happens). Even if an emergency happens, the individual is unlikely going to be listening to this radio. In this "individually owned" scenario, it's also unlikely any pattern work is going on.
In other situations, these may be owned by a flight school or a private group. I'd have to imagine in this case - they select one of the common CTAF frequencies also published in this table.
General Note: The owners of these opt to chart them for your safety in case of an emergency, and not much else. Some may be more generous and allow you to reach out to them with advance planning in certain limited circumstances. The bottom line is to realize that these fields are private for a reason.
FAR's specify that private airports can not have a CTAF frequency assigned. The most correct answer is MULTICOM 122.9 as mentioned above, per AIM section 4-1-11, table 4-1-2. As far as nothing much happening at private airports, I've been flying from one for over 15 years and we routinely have 10-15 aircraft based there. Yeah, there is radio traffic. Now, since you're not supposed to land at a private airport without permission, you should just ask what frequency when you ask for permission to land, right? There are a few private airports that still use 122.75 (air-to-air) which at one time was also for them too, but the FAA dropped that frequency assignment from the FAR/AIM a few years ago.