You can find out from the DVLA and from the owner of the car. By law, the owner has to tell you the extent of damage that the car has suffered or you are entitled to a refund. If the owner is vague about it or says that he/she does not know, it was repaired before it was bought or not sure what the cat is then you really need to give it a miss.
If the car is a Cat D then you should be fine.
If the car is a Cat C then you need to walk away unless you have the knowledge yourself or have some one with you that knows the ins and outs of cars, chassis and running gear.
You would be better off to do a HPI check on the car and go for a Platinum check or a gold check which should cost no less than £20. Before you do the check confirm that the HPI company includes insurance on the check to an amount that exceeds the value of the car. Sometimes even the HPI companies mess things up and the car turns out to be a minger. In these cases, the insurance covers you for a refund of the total loss.
It is always best to do a HPI check even if the car is costing you £1000. There could be a marker by the Police on the car or anything.............
El