When you arrange to view the vehicle, ask the seller not to warm the car up before you get there.
Before starting the engine, check fluid levels are correct, engine oil, coolant, brake fluid, etc.; low fluid levels can indicate slap dash maintenance. Before the seller starts the engine, make sure it is cold, if it starts well from cold and warms up smoothly, this tells you the IACV is working correctly. If it is already warmed up when you get there, is the seller trying to hide something? Once warmed up, switch on the a/c and check the engine maintains a steady idle, if not the EACV may need replacing. this also allows you to check the a/c is blowing cold. If the a/c isn't blowing cold it may just need a recharge, or a complete overhaul; I've not done this yet so not sure how much to ask off the price, someone with more experience of a/c might chime in.
Check everything works, lights windows, sunroof, dashboard switches. Sometimes the windows can be a little slow to roll up and down, this can often be fixed by simply spraying silicone lube into the window tracks. But it can be a good bargaining point if the seller doesn't know that, tell them the window motor is on the way out and ask for the cost of a replacement to be taken off the asking price.
Under the bonnet, look for a line of oil spray on the underside of the bonnet, to the right of the centreline above the auxilliary drive belts; this indicates a seal on the power steering pump has failed. The seal can be replaced easily enough, but you might negotiate a cost of a replacement p/s pump off the price.
Check the sump plug hasn't been drilled for locking wire, if it has been drilled this can indicate the car has been on a race track. This is not all bad news as people who track their cars do tend to look after them, but it does give you another bargaining chip to ask for more money off, say you're worried that the car has been thrashed.
Look for rust on the brake discs, if they are rusty it has been stood for a while, the brakes can seize up and may need overhauling. Ask for the cost of discs, pads and exchange calipers to be taken off the asking price.
If it's bone stock and well looked after (regular oil and filter changes) the standard engine and transmission are very robust, it is a Honda; some people on here are putting 300bhp+ through the standard transmission. If it has been allowed to overheat at some point the cylinder head could be warped; just do the usual checks for water in the oil. If you want to be really sure, take a compression tester with you; you're mainly looking for compression to be equal across all the cylinders, if one is much lower than the others it obviously points to a problem that you will almost certainly have to take the head off to fix, ask for lots of money off. Compression information is at the top of this page. http://media.honda.co.uk/car/owner/media/manuals/PreludeManual/62sf100/3-2.pdf
Body wise, check all the panel gaps are even as uneven gaps can indicate accident damage. If you do find uneven gaps, check the chassis rails for ripples, they should be smooth and straight. Open the doors and try lifting them up, if the whole vehicle moves when you lift the door that is good, if the doors clunk around and you can feel the doors moving without the vehicle, the door pins need replacing; not a difficult fix but more money off. The real enemy though is rust. A common area is around the rear arches and I've also seen them rust underneath, in front of the rear wheels, at the rear end of the sills. Also check the boot (trunk); lift up the boot seal and check around the lip for rust, particularly near the boot lid latch. Check in the spare wheel well, if the drains get blocked water will collect here and obviosly cause rust.
While you are under the vehicle checking for rust, have a look at the shock absorbers, if they a re leaking they will need to be replaced. Also check for broken road springs (can be difficult to spot if they break right at the bottom).
When you test drive the car, listen for clunks, bang or knocks as you are driving along. These can indicate worn suspension bushes or joints; either will need replacing.
When you get a bit of open space, put the steering on full lock and drive in circles, in both directions. Any clunking here (like a regular dunk, dunk, dunk) indicates a worn UJ. Negotiate the price of exchange axles off the asking price.
I've had experience of four different 3G's, mechanically and electrically they have all been pretty spot on, but rust has been the killer for 2 of them. If anything else comes to mind I'll add it in.