Let your scouts scout

And then when they bring up their list of potential players i always watch the scout reports (in that list). Then sort on Potential Ability. If a player have 3 stars now and 5 i the future (in example) i look closer to that guy, let him scout for 1 or 2 matches, but basicly it's best to look at his stats yourself. It depends if you have loads or money or not as well. When you are on low funds you might get players that are 18 and have 1 star but potential 4/5 stars. Meaning he's cheaper. But i suggest you make the decision yourself, as you said they most of the time don't live up to expectation.
When i actually gonna sign new players i prefer to do my own scouting by bringing the whole list of available players up and look at what i could afford or who is willing to join my club. This lists (at least the star rated players) expands when you're scouting. So in that way it's still useful. For the rest you just need to be lucky if a future prospect is really going to be that great. Otherwise, just look at their abilities and compare them to your current player on that position. Never trust only his advise