VPS servers usually are configured with software called virtuozzo, most VPS servers have 4 users on it or so, you get complete control over WHM, apache, php etc, and can actually optimise / break the server configuration, but virtually, rebooting the server won't do a hardware reboot, it most likely refreshes your virtual account.
A VPS will act and behave as a dedicated server, but whereas on a dedicated you are the only user on that server, on a VPS you are on a semi - shared environment, and only have control over your virtual account.
You can always ask the provider what they do but I think you will find most companies put around 25-30 people per Dual CPU system with most VPS techiologies. Of course the more RAM you have tehnically the more people you can put on a server
If you want to know in non-technical term, it is just like sharing room with your friends and paying the rent in a shared manner, so your maintenance work is also limited and you are going to have all the rites for your needs.
I hope this will give more easy idea regarding VPS.
I think first you should ask who you got it from. Then if you don't believe them you can try and look at the file system. For example I think virtuozzo has a very distinct file system signature. YOu might also be able to tell if the mem/HD was amazingly low for a dedicated server (256M/10G) .
I know on a product like VMware there is probably no way you could tell without poking around in memory.