There is a hand-pump-up device which you can use to pressurize the cooling system. Car and tractor dealerships almost certainly have one near you. Do you know a mechanic who could borrow one for an evening or a weekend?
Gravity always functions consistently. Clean off the area, dry it, pressurize the system and wait for a leak. When you see it, follow it UP, dry it off, wait for a drip, follow it UP, use a mirror or even your dry finger to identify a leak.
If I was a betting man, I'd bet on a frost plug on the side of the block. You might be able to demount the water filter and move it slightly without disconnecting the lines, giving you a better view.