The market is flooded with VPS (virtual private server) hosting options. It seems everyone and their mother has a overloaded server in his/her closet. Enterprise options always seem priced insanely high, which make the ones that are cheap and claim enterprise level seem shaky.
What do you look for in a quality VPS provider (language support, 24/hr tech, etc), and how if at all do you check their credibility?
I've tried quite a few of them. The only one that I can recommend wholeheartedly is Slicehost. They are incredibly good at what they do. I have many clients running on their systems.
Check credibility on forums like http://www.webhostingtalk.com/
I am about to purchase a VPS and after some research I selected http://www.servint.com/
I use rackforce. I havnt had a problem with mine, but then i dont have anything large scale on it. One thing about them that is good is they are on a borderline of two powergrids and backbones.(so i have heard)
Just stay away from Webserve.ca. They are HORRIBLE, horrible support, they mess things up, slow to resolve problems. and i heard they are a reseller. I tried them a while back when i just needed something quick and didnt do any searching. Bad bad bad.
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:
Those are things that I look for.
I think one way is to look for ones that reputable sites use. For example, I learnt about Slicehost through Refactor my Code using it, and I love it. :-)
The best way to find good or bad references about a hosting service, is always googling and foruns. I always look for a good support and how flexible it's service provider is. One that i have as a development and staging server is in the "A small orange" http://www.asmallorange.com/services/vps/. They have even a developer package, that, is what i use. But i started there getting a simple cheaper hosting plan (25 dollars/year) and I checked their support, their uptime and when i felt confident in their service, i've got a VPS. I recommend it.