With a straight edge and feeler gauges.

At best, they're a temporary stopgap measure until you can get the gasket replaced.

Depends where it's cracked. This was a common problem with the 6.2 Detroit Diesels that GM used from 1984 - 1993. You'd have to magnaflux the head, find the cracks, drill a hole and tap it, fill it with a bolt, cut off the remainder of the bolt, then smooth it all down. Not someone you really want to "learn as you go" on your own vehicle.

Please re-write this so it makes some sense

Yes it can but you risk doing more damage internally if coolant is mixing with the motor oil.

Usually the best mechs. Light line does minor repairs heavy does the good stuff like rebuilds, blown head gaskets, etc

The intake manifold, exhaust manifold, and head must be removed. This is a major repair. On a 26 year old vehicle you might as well do a complete engine rebuild once you remove the head. The vehicle may not be worth the cost of the repair.

Without knowing the make, model, and year, it is hard to say. The national average for a head gasket repair is $1144 to $1468. It's generally less than $100 in parts, and the rest is in labor. You can find estimate calculators online.

One example: http://repairpal.com/estimator/head-gasket-replacement-cost

There are also many others.

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