Research the model, find out what faults that specific bike has. It's a triumph so it'll have its foibles most likely. Nice bikes though. Don't be put off with lack of dealer servicing, frankly my bike's better serviced in my garage than most dealer serviced machines and triumph owners tend to like spanners. But, if it doesn't feel right, that's not usually a good sign.
Oh, I get the impression that you're not that familiar with bikes, or the mechanical side at least, try and take someone who is- I'm quite an aggressive fault finder and as soon as I can find one issue, generally the seller opens up more. And there's ALWAYS something, just like cars.