Hi so as any fuel passing through an injector over time makes the holes in the injectors wider it must follow that after as many years as this engine has been working it is no longer spraying fuel in so not burning so efficiently as it would with new injectors.
You take the harness connectors off and do a resistance check on them. Then if they all read the same you only need to worry about if the spray pattern is ok. To check that you remove them from the motor and activate them one at a time with a scan tool while there is fuel pressure going to them.
I wouldn’t approach it without a manual. library might have something. In general, look at any seals If you can pressure it up then you could look at the spray pattern (withdrawn from the cylinder) or if there is dripping when turned off. But all of this requires caution. You must first disable the coil. If there is a spark present you could have a flash fire not the , best thing for your complexion.
The Mechanical Test is listen to them clicking. I use a long Screwdriver, but you can use anything including any metal tube or rod. If one doesn’t click make sure the wire connection is good, Them I would pull the plug to it and check that one terminal has power. Then check for Pulsed Ground with a Noid Lite. To check the Spray Pattern, you’d need to pull the Fuel Rails, which you probably won’t do.
if its tbi you can probably see them spitting and dripping fuel …