What should I look for if I want to buy a slab on grade house?

Where is the house, and do you know how old it is? Those two things would offer a first indication of how well it is insulated, depending on what the Building Code requirements were in that location at the time of building. My first recommendation would of course be to have a building inspector look it over before making a purchase. Below are some things you should put an eyeball on - 

I would check for cracking, specifically along the edge where the footing would be to see if there has been any significant shifting or settling. All concrete will crack, that's just a fact of life, but I would more be looking for structural reasons rather than aesthetic, so checking if there is a shift in the level near cracks rather than just normal cracking from expansion and contraction. 

Check the exterior above grade portion, you may be able to notice any shifting issues there as well, you may have some indication as well of the insulation levels, and if it has been damaged in anyway. 

Slabs also tend to be closer to the ground that a standard foundation, so I would inspect the bottom of the siding to see if there is moisture damage. 

I would also ask for any building details that may be available; and if it has a radiant floor I would personally want to see the utility bills to know what a winters worth of heat would cost. 

And a last piece of advice - don't be instantly leery of it simply because it is a slab. As long as it is in good shape, I would consider that an asset compared to a basement.