What are signs of of bad intake gauset on a 99 chevy blazer

Depends on whether its leaking coolant or air, since both run through intake. if leaking coolant, you will see coolant leaking from either the front or rear of the intake itself. it can also leak into the engine, causing coolant to mix with the oil. if its leaking air, usually a small leak will present more when the engine is cold, with rough running or high idle. a large leak will run rough all the time, but should be able to hear air hissing if large enough. a vacuum leak from the intake will sometimes present as a check engine light, with codes p0171 and/or p0172, depending on whether the leak is on one side or both.

Check the oil cap for oil/coolant mixture usually the intake gaskets go bad internally near cylinders 6 and 5. you can also coolant pressure test and visually see a leak or hear air in the crankcase.

Rough running, one quick way to find out is take a bottle of water, and pour it on the intake, if it is leaking it will smooth right out and purr like a kitten for a minute, then back to rough. WD 40 will do the same thing. The water will not hurt anything.