Cars

1) It’s leaking oil from where the cylinder head is placed on the block (oil will be streaming down the side of the engine block).
2) You are losing power under acceleration and the engine has at least one cylinder that fails a compression test (this is indicative of several problems, not just head gasket failures).
3) There is water in your oil sump. The oil will have a grey-brown color when you drain it.

That’s all that I can think of. #1 and #3, you can check yourself; #2 requires a compression gage that fits into the spark plug hole.

HTH

Usually when the head gasket is bad the engine temperature is very high because the engine oil is mixing with the radiator’s water/coolant and cannot properly cool the heat of the engine.
You can easily check if that’s happening: Locate the engine’s oil cap (that hole where you fill/add the engine’s oil) on top of the engine unscrew it and if you have a bad head gasket you’ll see some kind of foam or a brownish sediment under the cap, that means the water and oil are mixing. But if the engine’s temp is normal, perhaps you’ll need to consider another problem with the car and not the head gasket.
Cheers!