If it's been raining and you don't have a flue cap it's conceivable that it's rainwater, but don't count on it. Condensation, not so much- can't happen.
Near end of life older boilers will leak when cold, as the seams between the cast iron sections have expanded & contracted thousands of times during it's life, stretching the bolts that hold it together, warping the plates, wearing out the gaskets, etc.. If it's 20+ years old this is a likely scenario, and it may be time to start scoping out replacements.
To figure it out, somewhere on the system there should be a pressure gauge (sometimes on the boiler itself or inside an access panel, sometimes external to the boiler, sometimes it's an integrated temperature & pressure gauge) and it should be in the 12-15psi range. If the boiler has an auto-fill valve keeping it there, it could leak quite awhile with no drop in pressure. But between the auto-fill valve and the water supply there is usually either a ball-valve or gate-valve for turning off water to the system. Make a note of the system pressure, turn that valve off, and see if the pressure drops over the next few days.
If it loses pressure, it's leaking out somewhere. If it's leaking at a plumbing connection the repair can be fairly straightforward, but if it's between the boiler's heat exchanger plates, probably not. You may have to eyeball the thing pretty carefully to figure out exactly where it's leaking, maybe even turn off the gas and use a mirror to peek at the fire-side of the section plates looking for unusual corrosion at the seams, etc. But if it's not at the (usually exteior) plumbing connections, it's probably on it's last legs.