i would try pulling out the thermostat and throwing it in a pan of boiling water to see if it opens if it does then you could take the oil cap off and see if theres any milky residue foaming up in your clinder head-HINTS-leading to a blown head gasket, or you could check the oil for any sign of coolant mixing with your oil also leading to a blown gasket. but your best bet before assuming its blown is to check the thermo first.