The most common sign of a blown head gasket is that you have white smoke coming out of your exhaust when you start the cars engine.
The white smoke may continue as well when you accelerate but sometimes it can be hard to see the white smoke when driving down the road because the air around the car will push the smoke away.
Be sure to check your coolant level as well when you see white smoke coming out because it's a sign that your engine is burning coolant and you either have a small head gasket leak or a severe head gasket leak.
If the head gasket is leaking really bad then your engine will almost always overheat as a result which can lead to engine damage.
If the temperature of your engine is going up and you're loosing coolant then your head gasket is likely blown if your coolant is not leaking anywhere else.
You can use some Bar's Stop Leak Head Gasket Sealer or other head gasket sealers to seal small head gasket leaks by draining your coolant and pouring in the head gasket sealer and then add water.
Then let the engine idle for around 15 minutes or take the car driving slowly around town for the 15 minutes and then shut the engine off and let it sit for 30 minutes.
Then check the water level and start it up again and see if it's still blowing white smoke and if not then the head gasket sealer has worked.
If it did then drain the water and add coolant again to keep the parts lubricated and to prevent your cars engine from freezing.
I used Bar's Stop Leak head gasket sealer in my car 5 years ago due to a small head gasket leak and it worked great and is still working great.