The interval is every five years or 74,500 miles (although this was revised to 60k on some models), so there should be evidence of a timing belt change. Where the book has been stamped, it should also say what service was carried out (eg level two, or even 60k service) - which you can double-check to see what it includes. Don't be fobbed off if the dealer says it's good for another 5k - the belts perish over time as well as through use. It's also critical to get the belt done as depending on what kind of engine it's got a snapped belt could lead to the pistons clattering into the valves and the engine being scrapped. If the dealer can't satisfactorily prove to you the belt has been changed (with actual evidence as opposed to verbal reassurance or excuses like 'we're still waiting on the history file), then you have two choices. One is that you can walk away. The other is that you tell them you won't be buying the car until they've replaced the belt.