Everything agmantoo said, with emphasis on walk-away-from-aluminum-wiring.
Also, have a look under the house, look for puddled water, which is not a good thing. (Leads to mold damage and may be a sign of leaking pipes.)
Check the house for rodent infestation; mobiles can be really bad about this. Look for rodent droppings under the kitchen sink, etc. Rodents can be killed; the damage they leave behind can be expensive to fix.
Check faucets, hot water heater, appliances, etc. for leaks, make sure everything works -- mobiles are made with the cheapest possible parts and appliances and things like exhaust fans (bathroom and kitchen), light fixtures, hot water heater, fridge and stove and dishwasher, etc. -- they're all prone to failure.
In newer model homes, 2X6 construction is much better than 2X4 construction. I've got a new 2X6 home and my electric bill in summer is around $130. (Air temps get into the 120's in the summer) My neighbor's got 2X4 walls and pays about $200 for a DW the same size.
Stand outside and have a critical look at the home. Is it crooked? Listing to one side or another? Walk around it and make sure everything's true and square. If it's sagging because the jacks have sunk, this can be fixed depending on the degree of sag, but I've seen drywall crack in the process. If it's listing to one side or another, it's probably not structurally sound.
If it's got metal siding that's dinged and dented, there's no fixing it short of replacing the siding. (Around here, people stucco them with good results.)
No skirting? You want skirting. This isn't just there for cosmetic reasons, it also helps to prevent the wind from getting under the home and flipping it over.
On that note, is it tied down? Is this in an area where it NEEDS to be tied down?
Look at the porch, if it has one. Is it well made? Sound? Porches aren't cheap.
Has anyone put an addition on it? I've seen some real nice additions put on a mobile home and I've seen some that are of the tarpaper-and-pallet variety. If there's an addition, though, there's a good bet that it wasn't done professionally and the wiring and carpentry may not be up to snuff.
Otherwise, look at the usual house stuff -- do you like the color scheme, the floor plan, what's the condition of the carpet, etc.
Which way is it oriented? This is important in hot weather climates. You don't want a mobile home that has the long side facing west ... particularly if it's an older singlewide, you'll cook in the summer.
And if everything looks OK THEN HIRE A BUILDING INSPECTOR. But it's quite possible to eliminate a lot of houses before you even get to the building inspector state.
Oh, and even a well maintained older home isn't worth much.
Leva