It's the intake.. I've run into many of these that I was only able to verify by replacing the gaskets.

They tend to leak when the engine is cold, you might try the same check on a cold engine on a cold morning.

If your engine is SOHC (Single OverHead CAM) then replace both upper and lowers. There are no coolant passages in this design and once you have the upper off, the lower is a piece of cake.

If your engine is OHV/EFI (Traditional Push-rod style) then you only usually need to replace the upper.

Good luck.