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If they're plugged, one or more cylinders will misfire due to lack of fuel. If one sticks open, typically due to varnish buildup, fuel pressure will bleed down when the engine is off. That results in a long crank time to get the engine started. If one is shorted, it may not open to let fuel in, or the Engine Computer could shut down to protect itself from excessive current. That last one is pretty rare.

Symptoms are generally pretty subtle such as a mild hesitation or increased tail pipe emissions. When an injector doesn't spray enough fuel, there will be excessive unburned air going into the exhaust system. The oxygen sensor will detect that air and report a lean condition to the computer. The computer will respond by commanding more fuel from all of the injectors because it doesn't know that just one injector is causing the problem. While all the other cylinders get too much fuel, there will still be that unburned oxygen from the lean cylinder. Oxygen sensors don't detect unburned fuel, just unburned oxygen. You might smell the excessive fuel at the tail pipe, but the computer still thinks the engine needs more fuel.

GM has a big problem with mismatched injectors that flow different rates of fuel. At higher mileages, that can result in misfires and a rough running engine. More commonly it results in high emissions. Chrysler buys their injectors in matched sets so they don't have that problem. Most other manufacturers have some way to check their injectors so they're matched.