You could apply some constant current pulses and monitor the capacitor's terminal voltage: when the current is switched off, the terminal voltage will show some step decrease - dV = RESR * Iconst

Although I personally would rely on the capacitor's data sheet :)

To get a clean resistance value out of your measurement, you net to set the ac frequency close to the ohmic working point of your LCR equivalent circuit. Is the frequency to low, you are going to measure mainly the impedance of your capacitor; vice versa you get the value disturbed by the inductance. Moreover, with decreasing capacitor value, your required frequency is increasing. 100 nF requires at least some MHz Bandwidth. If you are not sure about the L and C values, you should sweep the frequency in a certain range and search for the phase transition. At exactly this point your measured impedance is purely ohmic.