A worn valve seat is very shiny (looks like chrome) but has black pits in it. When you lap a valve correctly, the valve grinding compound grinds the valve seat surface of the valve and the head. These shiny surfaces will turn a dull color. When the dull encircles the entire ring, then the valve is lapped. All the black pits must also turn the dull color.
There are course and fine grit grinding compounds. you will need to get a valve grinding stick £1, some paste £2.50 get course and fine , google valve grinding thers loads of vids etc showing how to do it propperly