Most virtual hosting platforms will have a trial period in which you can test out their reliability. They will also give you a list of their high profile sites on their systems. Most keep track of the traffic hogs as it's a great way for them to attest their own stability.
I would recommend Slicehost as I have been with them for over a year and love the control. They have an amazing panel in which you can console in, rebuild slices, and restart slices in an instance. They also allow a VERY fast and painless memory upgrade, bandwidth pooling (taking all of your accounts bandwidth into one large pool), and they allow lots of different Linux kernel OSes.
So to answer your question without sounding like a complete advertisement:
- Check about their remote capabilities to manage your VPS.
- Check out their largest clients and some big sites on their systems.
- Test out their VPS for 30 days or so and give their support a test!
- Check out forums where people talk about services (like this thread mentioning Slicehost 3 times already).
- Check out places and make sure people aren't complaining of overselling or crowding out servers. I know in a VPS world, things are sandboxed a lot more than shared hosts, but it's still nice to know they can handle loads.
- Check out the abilities to move servers or add more memory to your VPS.
Those are things that I look for.
