This could be one way to avoid a capacitor.
Start a timer, then start sampling the voltage as fast as possible, till the peak value is reached, this can be found when the sample values start to decrease, then stop the timer, record the time interval (in msec), now take the next sample after elapse of the time interval that was previously recorded.
The time interval represents the time taken for the voltage to reach peak value and this should be a constant, alternatively you continue sampling irrespective of the time interval.