Visual inspection is the cheapest and maybe best way. Check underhood along fuel lines, along the underside follow the fuel lines back to the tank; check the gas tank grommet at the filler pipe, and the filler pipe itself. Some of these have a rubber section to the filler pipe that can degrade. Check both with engine on and off, and with tank nearly full.
Look for a dried stain from liquid fuel. Fuel vapor leaks don't leave a stain and are much more difficult to find. You may have an underhood label with the evaporative emissions line routing to use as a map.
A front to back sniffing test in a closed area to find what may be the strongest area of odor may be best. Obviously dry rotted rubber hoses and components and rusted out metal lines/tanks are also odor sources. Liquid fuel leaks may still be wet and easier to find.
Many tank rust out leaks are found up on top of the tank under the tar paper mat. Gasoline will dissolve the tar and undercoat and leave it sticky or slimy.
I wrap my fuel lines from the tank to the intake with paper towels and then turn the key on. If there's a leak under pressure at key on, it'll soak the paper towel. If I don't find it that way, I start the engine for about 30 seconds. Sometimes I get lucky, sometimes I don't.
A hiss or puff when releasing the gas cap isn't really a valid test. The fuel tank is vented through the canister to the air for evaporative emission control. The activated charcoal inside the canister traps the hydrocarbons (HC) for later burning after the engine starts. If a line is blocked or pinched, you may have pressure build up inside the tank.
For evaporative (vapor) leaks, we have a smoke machine at the shop that blows cool vegetable oil smoke laced with UV dye into the evaporative plumbing at 1 or 2 psi. In a closed system the smoke has no where to go. This is good. If the system has a leak, smoke will be seen at the point of the leak. The UV dye is for the more difficult leaks that can't be easily seen. Under a black light the crack or pinhole glows.
For liquid fuel leaks, the fuel pump moves the fuel forward to the injectors under pressure. Any point along this path can show wet with fuel. 1991 may have the fuel pressure regulator at the fuel rail and return unused fuel back to the tank. This path can also leak.
Locating your leak source is the first step.
My 97 GC witha 3.3 had a leaking fuel rail there was a recall on it and the dealer fixed it free. It was more like seeping than leaking...but when even a small amount of gas leaks over the warm engine it instantly vaporizes and the smell if quite strong. where it leaked/seeped was right at a fitting. Not sure if the 91 have had similar problems. I also had a very small leak in the fuel line that is part of the fuel filter asby, it was just a pin hole and it .
Check the fuel lines clamps on the gas tank. I had an '89 where the clamps on the fuel hose that came over the gas tank
rusted out. Once the spare was out of the way you could see a very shiny spot where the fuel was spraying on the tank.
While there was no recall for the 1991, the fuel rails can leak on these engines.
Besides the fuel rail, other places I'd look would include the filler tube or the tank (especially on the top) for perforation if it's a metal tank.
When your driving the car or sitting at a traffic light, can you smell the fuel in the ventilation system? If you do smell fuel through the ventilation then the leak might be under the hood.
I have a 92 Plymouth Grand Voyager LE with the 3.3 liter v6 and I have had to replace the fuel pump and the fuel tank because the pump failed and the tank had a pin hole leak from rust under one of the mounting straps.
I now have a fuel leak on the fuel rail where that cross over tube mounts on the end of the front cylinder bank. And 92 is not covered under that recall. Also the fuel rail is no longer available.
Most likely it is the connections from the fuel pump to the return line if you don't have an active leak, or else it may be the fuel rail if your van is a 3.3 or 3.8 v6. So first check for odors or leaks under the hood while the van is running if you have a 3.3 or 3.8 - you'll see and smell the leak up on top of the engine near the manifold - the fuel rail runs right underneath. You can see the front one, it has a schrader valve for fuel pump pressure testing. There is another for the rear 3 cylinders.
Even if you have the 4 cyl or the 3.0, give the engine bay a good sniffing and inspection with a bright light. Better safe than sorry.
My 1994 had an odor anytime it was under 1/3 tank until I changed my fuel pump, then the odor went away. The fittings at the top of the pumps tend to crack and leak (they are plastic).
Check for a leak without dropping the tank by idling for a few minutes, then pulling onto a sharply sloped road or driveway (concrete or asphalt so you can see drips), jumping out and looking underneath at the back of the tank on the driver's side. You may be pooling a bit of gas on top of the tank, which then will run off when you park on the slope. Even though it doesn't look like much of a leak, they stink to high heaven.
Make note of when you notice the odor. If it is all the time, you have an active leak - if not, it is probably a small leak at the fuel pump return or vapor lines.
Good luck - and btw you can fix it cheap if it is a fitting leak by splicing in regular hose and clamps vs buying a brand new pump or a brand new set of pressure and return lines (assuming it isn't cracked too badly). My shadetree guy did mine for $80.