Hi Andrew,
If the suspension looks low at the rear you need to get it inspected by a garage.
Your other concern is whether the engine has been strained by the towing. There is no way to be certain. If you are buying privately then you can look around and deduce whether they were towing a twin axle caravan on holiday each year or taking a little trailer to the tip for garden rubbish. You also need to ask the seller how often they towed and how heavy the load was.
You should also check out our buyers guide for more common faults and wear items. See: http://www.usedcarexpert.co.uk/cars.aspx?numModel=66