I have 1995 ford ranger with 3.0 in it and I rebuilt the motor and I changed out the freeze plugs all around. I haven't checked for leaks yet. Is it OK to just pour water into the thermostat housing to check for leaks? Or is water just rust the inside?

 

Its fine to use just tap water for a short time
Yes, tap water has minerals(salts) in it so does promote rusting but only over months of use, not days or weeks
So you can run the engine and drive the vehicle with just water in it, so it gets up to 14psi pressure, which is best test for leaks
Engine with properly working cooling system will not overheat with straight water

Coolant has anti-corrosives that prevent rusting so once you are satisfied there are no leaks then drain the water and add coolant, you don't need to get all the water out just as much as you can by draining the system, pre-mix coolant is 50/50, 50% water, 50% coolant, so it already has water in it, distilled water but water none the less, lol
Coolant also has Water Pump Lube, for the water pump bearing, again a few weeks without this is not a big deal

Remove one heater hose at the firewall to refill the system, this lets the air out of the engine side of thermostat, when water/coolant starts to come out of the hose put it back on the heater core

Swap heater hoses around at the firewall every 2 years, this reverses the flow thru the heater core and will keep it cleaner and it will last longer