First, inspect for cracks, and evidence of coolant leaks, and other damage. If you find cracks, check with an auto machine shop to see if it can be repaired. If not, you will have to buy another head. Coolant leaks are visible by orange-ish stained areas.
Now, using a straight-edge, and feeler guage (as shown above), check the surface for warpage. If the warpage exceeds the service limit (0.002" [0.05mm]), it may be resurfaced at a machine shop. My advise, if you have the head off, and have anoher car, have it plained and ported (exhaust side ported AND polished) for a power boost. The intake side can also be ported, but must not be polished. Other power enhancements - bigger intake valves, thinner "performance" head gasket. . .
There is no specification in the manual on how much of the surface you can shave before you can't go any further.
While the head is off, check your Block for cracks and other damage.