You won't be able to see it if it leaks

Do the following to find out

1. Put all the valves back to where they belong (make sure each of them goes back to exact the same spot it came from) and make sure they are seated properly

2. Align the cylinder head such that bottom surface of the valve(s) you want to test is level and then pour some engine oil to the surface. If the oil leaks thru the seam badly, the valve has a leak problem

You can try to fix the leak with a special tool and some kind of sanding past (I actually don't what it is called in english)