What Ive used in the past is "engineers Blue" and a machined block of steel(perfect flat edge)
Have your head all nice and clean and check the obvious parts with a precision steel rule,
then smear a tiny amount of the blue on the machined block of steel, and rub in figure of 8's over the head, the blue will stick to any high points and will not stick on the low points.
When you look at the head after a complete going over, you will know whats high and whats low,
im in south london, any trouble or if you want me to do it for you let me know
chris