I am writing about the sites where you have to make a deposit before buy and the site offers to act as a MM in deals.
How could I know when they are trying to scam me? In general, how do I know if a MM site is trusted?
Well its too obvious to those sites that are trying to scam you, if the offer is too good to be true, that you can earn with a huge amount in just a few couple of time. And before risking any money from your wallet try to search in google ('the site's name' review) and read all the reviews there with different people so you can know if this sites are legit.
Checking feedbacks to a particular investments isnt always works and some of those feedbacks are from fake people which you will really need to investigate more and as you said checking sites status/name and you should really seek for the legit reviews and not to those fake ones because if you just let those positive feedbacks of fake people then you will surely got hooked into it.