There are obviously many such sites around these days, and as you say it's not always clear which are good or bad, but MoneySavingExpert lists a few such sites, a tool that helps warn of questionable sites is WOT (Web of Trust), it's a basic traffic light system that attaches to your browser, it is not a replacement for common sense etc., but it can show things you may not have been aware of, but it's not always the site directly that causes warnings, sometimes a page can contain a link(s) to other sites that could be bad.
But when looking for something you have no idea about, the best way to go is search about it, so just like Google gave you results for streaming sites, Google for reviews of each site that interests you, that should help a lot in deciding which the good and the bad are, it also goes deeper, it will also show you the sites reputation that way, just because the site may be a "safe" site from any nasty's, it doesn't mean their customer service isn't bad, another thing to help, any site\download wanting to load a toolbar, you need to ask why, often it's a 3rd party tool that they get paid for installing, anything like that you do not want, so watch out for those tick boxes when installing, hope this gets you on the right track anyway.