I know not everyone has a machinist straight edge in with the tools they own.
This is not the most accuate check
But it should be close enough to help you decide if a MLS will seal or the block needs to come out and me milled flat.
20" or longer extention, 1/2 perfered.
Decent set of feeler gauges
.0015, .002, .0025, .003
A good know flat item, a piece of glass or mom's glass top table work best.
Pull out the .0015 feeler gauge.
Lay the extention on the glass, make sure the square drive end hangs off the edge.
Take the feeler gauge, and check the extention for bow.
Spin the extention untill you can not get the .0015 feeler gauge under it.
Once the feeler gauge will not go under the extention, take the sharpie marker and mark the top.
You are now read to check your block, head, intake mani or anything else you need for warp.
For Head gaskets
MLS if the .0015/.002 slides under, get the part milled, or go to a composite
Composite, if a .003 or more slides under, get part milled.
About any flange with a warp of .005 or more, needs closer inspection, and a good chance of being milled