I think that's a fair comment when talking about us, as fans watching a bit of the game on TV or looking at the goalscoring charts or whatever, but it's literally what scouts are paid for. Obviously you'll always get ones that you call wrong, but the vast majority of calls made by scouts should be at least relatively sound (ie they might not turn out quite as good as you thought but they aren't a complete flop). I do some cricket scouting and I would back myself to go to a tournament and be able to pick the 5/6 best prospects there, regardless of the standard they're playing against or for. As I say, it's not an exact science but it's really not just a lottery, you can identify key traits if you know what you're looking for. Just looking at the example you used yourself, van Nistelrooy was signed by United for a club record fee when they were one of the very best sides around. Janssen was signed 16 years later for less money, by a Spurs side who were just starting to break into the top 4, with a view to him being back up to Harry Kane. I think it's pretty clear that the professionals could tell which one of them was better, even if we couldn't.
