How can I check for a water leak in a 2001 Eclipse Spyder?

The most common leak is at the passenger door around the outside mirror area. You can close the door with a dollar bill put between the weather strip and the mirror mount area. Try pulling the dollar bill out slowly and feel how much tension is on the bill. Now open the door, put the window all the way down and while standing on the inside of the door, block it with your leg while yanking the mirror mount inwards to bend it a bit. Try the bill trick to see if the door is tighter to the weather strip. You can test the seal using a garden hose and spray the area down good. Keep in mind that convertibles will have some leakage when going through car washes or when aiming a high power stream of water at the convertible top seals/weather strips. You might also check to make sure the door latch is adjusted so that the door edge is flush with the body panels.

the dry ice would work but you would have to lock it inside a air tight place to see where its coming from and it would take a good chunk to fill it where u would notice BUT the danger of this is the fact you can suffocate from locking it in a air tight room and u being in there to watch it and there being no air what i would do is do the dry ice idea in a garage with the door closed long enough to see where its coming from then open the door with a fan in front of it sucking it all out and then remove the ice ASAP