A great physique means nothing. It means they have found what works for THEM, but that doesn't not mean that they will be a good trainer for YOU.
I trust a trainer if he is well backed up with clients and credentials. I would not hire a trainer who did not have both of these. (learned from bad experience)
As far as taking internet advice, I take everything with a grain of salt, but also consider most peoples' advice, unless it's full-blown ludicrous. You can't be so bias to think there is only one way to do things. We THINK we know a lot about the human body when it comes to training and nutrition, but no one knows everything.
I think the important thing is just having an open mind, and being willing to accept advice from other people and experiment with different methods.
Another note on the physique thing...I've seen AMAZING nutritionists and trainers with rather average-looking physiques. Can't tell a book by its cover.