The best thing you can do is look for sites that offer booster packages for reasonable prices, with a reasonable conclusion time. Checking if the site has been around for long enough to collect some positive user reviews is also a good idea.
Let’s start with the easiest part: customer reviews, how much can you trust them? Well, it all depends on whether or not those reviews were made by real paying customers, and whether those reviews have been cherry-picked or moderated in any deceptive way. Usually, it’s best to look for what people are saying about a booster site on social media and look for reviews hosted by trusted third-party websites, like Google Review or Reviews.io.
As far as timetables and prices go, there isn’t a hard rule as to how much booster sites should be charging in order to be legit, but you can usually spot an Elo boost deal that is too good to be true. You can’t expect a booster to spend two weeks boosting your account from Iron IV to Diamond I and not charge you at least a couple hundred of dollars for their time, for example. If you’d like to know more about booster services and how much they cost, check out this site.