Cylinder heads

You need two tools. A 100% sure straight edge and a set of feeler gauges. Lay the straight edge on the head gasket surface, parallel to the length of the head. Pick a .004 inch (.1 mm) feeler gauge. Now use the gauge to check along the length of the straight edge between it and the head. You do not want the gauge to slide through.
Do this in three to four places and be sure to check the areas between cylinders closely.

Now measure diagonally across the surface as well.

Here is a good video on it:
https://www.youtube.com/watch?v=u4SzK_OjAkY

You are welcome.
There is a second method that I use as well but it requires a certified surface plate to use it so I generally do not mention it. Just be sure you have a good straight edge, do not want to condemn the head and then discover the straight edge is bowed.