Look at the water pump...if it's been leaking, you'll probably see a small rust colored line where it's been weeping out
Could also be the heater hose connector(s)...they are plastic and prone to cracking, especially on a truck that old. They won't leak until engine warms up...the crack will open up, coolant/water will leak out, then when engine cools off, the crack will close up enough to keep it from leaking.
