Three ways a head gasket can fail.
1. Cylinder to water jacket.
2. Cylinder to oil passage/pushrod passage
3. Cylinder to air
4. Cylinder to cylinder
No way to pin things down 100% without popping the head. But advise given so far is a good start.
Looking for bubbles in the coolant is a pretty good bet that the gasket is blown into a water jacket. But not seeing bubbles dont rule out the gasket.
Water in oil is also a good indication of being blown, (or cracked block). But clean oil dont rule out the gasket either.
If it were leaking to air, a simple walk around feeling and listening around the head should tell you.
After those quick visual checks, you need to do a compression test. Cause even if all of the visuals pass, it could still be blown cylinder to cylinder.
And even the compression test isnt foolproof. You may show a low cylinder or two, and think that you have a head gasket problem, so pull things appart and it could be something like a busted piston or ring in that cylinder.
So I guess to sum up what I am saying, there is nothing you can do to 100% verify if it is a head gasket short of pulling the head. But there are alot of steps that can help point you in the right direction.
Now if she passes all the tests above including the compression test, I'd be looking elsewhere for the issues you are having. Valve adjustment?