If it's a private deal, checks become doubly important.
Try all the locks to make sure that they do indeed lock and you have the correct keys.
Move anything that is on the caravan floor (awning for example) and stand there. You're looking for a spongy floor, walk away from the deal if you find this.
Ask for all appliances to be shown working, this can be a bind for the seller, but it's worth the time.
As Ian says, check for damp. If you don't have a damp meter (more than likely) trust your nose and eyes. Look for discoloured panels everywhere including below the bed boxes. If the caravan has been sprayed with air freshener or anything that might overpower a damp smell, be suspicious.
Look on the outside, are there any trim pieces missing (hard to source) or body damage. Small dents are worth noticing because although they may not affect water ingress they are a bargaining point.
Ask for a short tow of the van, you're looking for correct operation of the brakes, if you feel the caravan banging into the back of the car you will know attention is needed to the caravan brakes.
Make sure that the seller is the owner. A 1990 van may not be registered with CRIS, make sure you have the seller's home address.
These are basic checks; taking along someone that uses a caravan is a good idea. Apart from that, if all is OK,