How can I test fuel injectors

The nozzles are buried in the lower intake manifold. The tests are done very rarely, and require specialized equipment. What kind of problem are you trying to solve?

The truck has white smoke out the tail pipe that smells like fuel. the owner of the truck thinks the fuel injectors are stuck open.

Dandy. Throw a fuel pressure gauge on it. If an injector is stuck, which is extremely rare, the pressure will drop as soon as the engine is turned off. It will also have an unusually-long crank time to get it started, and that leaking fuel will show up in the engine oil.

White smoke is a sign of burning engine coolant, typically from a leaking cylinder head gasket, but GM has had a lot of trouble with intake manifold leaks. I would look there first if the level is dropping in the coolant reservoir.

Also, be aware that water vapor is one of the byproducts of a properly-working catalytic converter. There can often be water dripping from the tail pipe or the drain hole at the back of the muffler. The clue that it is normal is the puddles will be clear, not green or red.

Can a smoke test find out if I got a manifold leak? If there was a coolant leak that would mean it has to be the lower intake gaskets ? cause im assuming the upper gaskets wouldnt leak into the engine or i could be wrong

The clue is if you're losing coolant.

You can use a smoke machine to inject smoke into a vacuum port on the intake manifold. That won't tell you if there's a coolant leak, but if there's a leak where the manifold runners connect to the cylinder head, you'll see smoke sneaking out. If you suspect coolant is being burned in the engine, add a small bottle of dark purple dye to the coolant, then check a day or two later with a black light. The dye will show up as a bright yellow stain that you can follow back to the source. Suspect an intake gasket if you find the dye inside the tail pipe.