From what I know, its call the RNG audit not RTP audit.
RNG (random number generator) program on slots is responsible for determining the outcome of each spin, and RNG guarantees that the past outcomes do not affect the future results.
And this RNG usually tested by independent audit labs such as Itechlabs, Ecogra, etc.
For online slots, some of the gaming providers provide a variable RTP setting but the slots operator/casino will not able to set random numbers.
For example, slot A standard RTP is 96% and the gaming provider of slot A only provides an option to pick 94% or 96%, because of that the casino will not able to set it to any number beside 94% or 96%.
Also to change RTP, the slots operator/casino is not able to change it anytime they like because they have to request it first to the gaming provider or via the 3rd party casino software provider and it will not happen instantly.
Some of you might aware that the same slots have different RTP at different casinos, it doesnt mean that some casinos are cheating with lower the RTP but because its legal and they are allowed to do that.
I suggest to play only slots with the standard RTP from the gaming providers, how to check it?
Google it or visit the gaming provider site to check it then compare with the RTP of that slot on your favorite casino.
To check the RTP, load the game then go to Setting or Information. Any casino will not able to change any info in the information tab including the RTP info about that game.
For example,
Because of the reasons above, the slots operators/casinos dont need nor able to provide an audit of the RTP/RNG because the slots operators do not have a direct access to that program.
And Im sure many people are still not aware that they play their favorite slots with non standard RTP on certain sites.