I'd get a radiator pressure tester, even if I had to make my own. Pump it up to around 25 PSI let it sit for awhile and watch the pressure gage. If it goes down, it's obvious there is a leak somewhere. Don't pump too much pressure in there anything over around 30-35 PSI may pop a hole in your radiator, if you look closely you can watch the copper passages expand. I made my own tester (for my Renix), a short piece of old radiator hose, a plastic cap a couple of hose clamps and a tire valve in the plastic cap. I use a bicycle pump to pump it up, my bike pump even has a built in pressure gage. Low tech but it works LOL.
If it isn't external, it is likely internal, either coolant leaking directly into the oil or leaking into a cylinder and passing through the ring gaps to the crankcase.
I've pulled the spark plugs on motors with head gasket leaks and seen deposits on the plug ends that look kind of like tiny cotton balls (sometimes white, covered in a light coating of carbon, sometimes even tinted the same color as the coolant) or if the leak is really small, sometimes you get really clean shiny metal electrodes on the end of the plug (steam blows the carbon off the ends).
If you have a moderate coolant leak into the cylinders, when you start up cold and the condensation comes out of the exhaust, hold a clean white piece of paper towel just below the exhaust and collect some of the drops, if it is tinted the same color as your coolant, it's a likely tip off.
Lastly check the vent action on your surge tank cap (radiator cap), if it is stuck shut, it can greatly increase pressure and the amount of coolant being forced through the leaky head gasket.
Usually when a head gasket leaks into a cylinder it will pass coolant into the cylinder and will also pass exhaust into the cooling system. The coolant often bubbles and foams at the top.
I've drained the oil and put it into a clear plastic five gallon water jug before, let it sit for a couple of days looked to see what's at the bottom.
Or find a piece of plastic tubing that fits down the dipstick hole (after letting the vehicle sit over night) and when it hits the bottom of the pan, suck some fluid out. I use an old wind shied washer pump.
If the coolant level isn't going down and the oil level isn't going up, it's unlikely to be a very bad coolant leak.
I had one bad coolant leak (head gasket on a Chev.) that actually filled a cylinder with enough coolant to bend a valve, compression went down really low. Hot exhaust gases in the cooling system ran the engine temp. gage up pretty fast (and high).
