(1)out into the overflow jug then it evaporates away easily when hot or (2) combusted in a cyl & out the tailpipe. Borrow a stant rad pump kit & w it hot open the cap (carefully) & pump it to 15 lbs & see if you see/hear any leaks in the next several minutes & if none (& no more than a fraction of a lb of pressure drop) let it sit (overnight if need be) & if it looses it's pressure if you have emission testing down there let it idle/take of the rad cap & set the probe in the air space above the coolant and it'll detect combustion gasses in seconds. A new rad cap may fix this ASAP