Clean head thoroughly. Soak the surface between the valves with penetrating oil. Wipe off the surface. Sometimes you can see a little bleedout from the crack after you have cleaned the surface.

Of course a real dye-penetrant kit would be better, and a Magnaflux using the magnetic powder would be even better.

Now here's a problem that has been discussed from time to time: the engine ran with the crack before the engine was taken apart, what makes us think that it won't run now? Also, some of the cracks are shallow enough that cutting the head for hardened seats cuts out the crack. But you can't tell from the outside how deep the crack is, and you never know what a crack is going to do, except you can be sure that it won't join itself back together!
I'm sorry, this is not a good answer. But maybe it's the best answer there is at the time.

R.