Detect blown head gasket - '98 Rodeo

pete_in_sf

V-6 engine. Number four cylinder has compression down to 140 psi. All other cylinders fine. I go through coolant quickly, like every hundred miles or so it's halfway down. No other symptoms, except occasionally the check engine light goes on or off with no apparent pattern.

I suspect head gasket. Obviously, this truck is old and the cost of removing a head is daunting. Is there a way to detect whether the head gasket is the problem and rule out, at least probably, other causes without actually removing the head?