A machinist would have special tools to check warpage. At home you might use the cast iron (machined) surface of a table saw or spare glass pane. Head is spotlessly clean and place on falt surface. Try to rock it and/or look for any visible gaps. Use feeler gauges to determine how much warpage. Tempting to hand dress warpage away with medium grit cloth on flat surface, depending. But compression and valve clearance can be problems. If block is warped different fix as it should be parallel to cyl bottom(s) and is difficult to measure at home. Pro shops please edit / add to my remarks.
