First of all have a look at the condition of them, they are behind the radiator, and there are 2 of em. If they look all cracked/hard and flogged out, they'll be pretty old. On the longer lengths of the belt give em a push down and see how far they deflect (ie move) If they feel soft to push down, then they are prob. loose.
Good trick I learnt to tell noises apart, is grab a shortish bit of garden hose, and hold it next to the thing you want to hear, while you listen to the other end. It cancels some of the other noises out, and can pick up suspect bearings on tensioners and other bits. Warning- you might look like a bit of a retard
You only drove it to 3500rpm...?
Ummmm... go back and flog it. Trust me, they expect it. Just don't drive it like you stole it, or break any obvious laws. Noises will become apparent if you drive it harder.
They sell evo's, so they are use to people driving faster cars.
That being said- Northshore stand by their cars. Honestly, if you get someone like a workshop to look at it, they most likely won't have a clue what their looking at.