Measure the voltage across the resistor.
Power = I*V = R*I^2 = (V^2)/R
The last equation is the one you want since all you need to do is know the resistance and the voltage across the resistor (easily obtained without breaking the circuit, unlike if you measured the current). So if the voltage squared divided by resistance is larger than the power rating, you need a larger resistor.
Easiest way to test a resistor?
Step 1. Eyes. See smoke or fire?
Step 2. Nose. Smells like burning resistor material?
Step 3. Finger. Does your finger burn?