I have a '91 Celica with 175,000 miles on it, and as far as I know, the fuel filter has never been replaced.
If it hasn’t, what sort of problems could this cause? How do I tell if the filter needs to be replaced? Is it very difficult to do, or is it the sort of thing a mechanic ought to do?
not too often… if you dont have a performance car its not a big deal… i do it once a year.
but you, yeah… 175k is a bit… you should change it about now 
If your filter hasn’t been changed in 175,000 miles, I definately reccomend you taking care of this ASAP. I generally reccomend this to be done every 30,000 miles, although some will say that is overkill. On most cars it is a pretty simple job, especially ones with lock clips on the hose as opposed to banjo bolts tightened to 300ft/lbs (not literally, but they are a pain in the rear to get off never the less)
If you plan on doing this youself, I advise you to atleast pickup a Haynes on Chilton manual for your car to get a better idea of component location. If you want to look yourself though a few common places are: 1) Rear bumper cover 2) Under hood against the firewall 3) Under the car midway between fuel tank and motor (Fairly common location for Toyota’s actually, even the Supra =)) Also, before taking those fuel lines off the filter make sure you de-pressurize the system. To do this you will need to atleast remove the gas filler cap. Some cars have a fuel pump shutoff switch as well where you could turn the fuel pump off and then try to start the car for a moment thus clearing remaining fuel from the lines without actually pumping more into them. Again, Haynes or Chilton will get more specific for your car on this.
Also, before changing the filter it would not hurt to run a bottle on Fuel System cleaner through the system. Lots of name brands to choose from here, availble at any auto parts store as well as Walmart in the automotive section. You would want to dump the bottle into an almost empty tank (preferably while your at a gas pump) then fill up with some 93 octane gas. Drive until the tank is almost empty again, and then change the fuel filter.
If your fuel filter is bad it could cause quite a few issues such as: Hard start, hesitation, poor gas mileage, and more.
If you don’t change it, the crap that it filters out can actually jam up your fuel flow. Also, as it gets older it can start to disintegrate and put stuff into your engine you’d rather not be there. Also, it loses its effectiveness after too long.
As for changing it, it can be tough. The one on my old car was on the firewall, behind everything. It was a cast iron bitch to replace. Some are easier to access. All that’s required is taking the hoses off of either end and reattaching them in the appropriate leads on the new one. No big deal.
Hmm, I think I will be going to a mechanic for this. I can handle an air filter, but this sounds a tad bit more complicated!
I forgot to mention that you will definatly want to have some towels out there with you when you do this because no matter what, you WILL leak out atleast some gas when you disco the lines.
Check your car manual, usually 20,000 miles is about right…You can get away with not changing it as you noticed for long after that, same with the radiator water… but your car would last longer if you change more often than 175,000 miles…