For a reasonable estimate, if you look at the resistor datasheet, the ambient temperature at which the resistor allowable dissipation drops to zero will be the resistor temperature at maximum dissipation and ambient for that dissipation. You can adjust that to account for actual dissipation and ambient range.
For example, this Royalohm datasheet:
Shows a 155 degrees C temperature at full rated power and 70 degrees C ambient. So if your ambient is 60 degrees and the power half of of rated the temperature should be around 100 to 105 degrees C.
Note that the thermal resistance to ambient of surface-mount parts is quite sensitive to the layout and amounts of copper around then so this applies when you are using the ‘standard’ test layout. A well-heat sunk 0603 might be able to run cooler than a poorly heat-sunk 1206.