Fraudulent mixers are the worst type of fraud, since they are illegal in all countries so there is nothing you can do if you are scammed.
The only guarantee is that the mixer is reliable, otherwise if they steal your money, you will lose it forever and you will never be able to get it back again. You will not be able to file a complaint against them to any government agency.
So the only way is to warn users about these scam mixers before they fall into their trap.
There are other mixers who are dedicated to marketing their service and platform to the community and they promote in places where they can show their legitimacy one of these places is here in Bitcointalk and one of these is Yomix, in the past, there's only one mixer that actively promotes their service here, I'm glad we have two or three more, Mixer users will have a wide range of choice, use them and compare each service.