YouTube user NeedSec has posted a good tutorial video showing how to use Kalibriate-RTL, a program used to determine the frequency offset error of your RTL-SDR dongle. Every RTL-SDR dongle will have a small frequency error as it is cheaply mass produced and not tested for accuracy. This frequency error is linear across the spectrum, and can be adjusted in most SDR programs by entering a PPM (parts per million) offset value.
Kalibrate is a Linux program that uses GSM mobile cell phone base stations to determine the PPM offset, by using the GSM signals own frequency correction bursts. See the tutorial video below.