An ARF or Kit that is already constructed is only going to be as good as the person who built it, but overall it should be pretty trustworthy. Two easy things to check:

1) Is the horizontal tail stabilizer installed straight? Check this to make sure that the distance from the right wingtip to the right stabilizer tip is exactly the same as the left side. The further apart the two measurements are, the more poorly built the model is.

2) Check the two wing halves to make sure that the glue joint is still strong between the two halves. Check the wings also for damaged ribs.

If the plane is holding up well, was built straight to begin with, and hasn't been crashed then repaired in any obvious way, it's probably a good buy. I'd honestly rather get a well-built older model successfully flown by an experienced pilot than buying a new ARF and hoping the assembly line didn't short any glue on one of the critical spots of the model.