If we are talking about slots in online casinos, software is usually provided by well-known providers. I think if someone wants to sell their products in such a specialized industry as gambling, it has to be of the highest quality. I do not think casinos want to change such software to set the RTP unfavorable for players. Of course, these things can happen at little-known casinos where the software is self-made. Therefore, to avoid unpleasant surprises, it is better to play in large and famous casinos.
You have a point on that. As we can't determine its technology, we are just relying on the casino itself.
If they are reputable and considerably one of the top casinos, it is their responsibility to make sure they are also getting quality services from top providers.
Because if there will be vulnerabilities and other related-concerns, these will be detected by some players who know how to test the service.
So in this case, if you are a gambler, you are not looking at the reliability of the technology but the casino's reputation in general.