As a user of RTL-SDR devices for tasks such as aero monitoring, ACARS, ADS-B, or perhaps VHF maritime or satellite listening, you may sometimes question the sensitivity of your radio. The answer is more complex than, "how high is the signal reading?" To have precise metrics, it is necessary to connect the dongle to a test rig, measure the noise, and inject a weak signal to compare with the noise.
Manufacturer's data indicates that an RTL-SDR has fair sensitivity - enough for local and more distant broadcasters and typical aero / maritime mobile stations. With a quarter wave vertical or one wavelength loop, mounted on a rooftop, high altitude flights should be audible out to 200 miles. Boats or aircraft on an airport should be audible out to about 20 miles. Distances may be increased significantly, even without a preamplifier, with the use of a directional antenna.
Simply increasing your gain settings does not help. The problem is that the first RF amplifier (or the mixer) in a radio is what sets the noise figure. A sensitive radio will have a noise figure of about 1 or 2 dB on VHF and UHF. The RTL-SDR has a noise figure of 5 to 8 dB, depending on the frequency and whether you received a good unit in terms of manufacturing tolerances. While it is possible to set your device gain too low and lose the front end noise floor below other noise added by the mixer and IF amplifier, there is no advantage to set the gain beyond about 25 on the RTL-SDR. Too high a gain setting simply causes more overloading and spurious junk to be created within the dongle.
Replacing a minimal dipole, vertical, or single loop with a simple three to five element yagi antenna can bring weak signals out of the noise and extend your max range by a solid fifty percent (limited by the radio horizon). What you should observe is stations within range having stronger signals, then a sharp drop for any beyond the visual horizon, then fluttery and weak signals out to approximately thirty percent past said visual horizon. Whenever there is propagation enhancement due to tropospheric bending or scatter from aircraft, you can receive signals at greater distances.
After installing a directional antenna as high as possible, and using a low loss feedline to the receiver, station sensitivity can be improved adding a low noise preamplifier. The term "low noise" means the preamp should have a noise figure below 1 dB, and the lower the better. Install the preamp at the antenna feedpoint, or through a short length of low loss cable. Typically, high gain is not necessary - 15 dB is plenty; more gain than 20 dB is often a waste and probably will cause troubles with overloading. If you do experience problems with overloading, especially from out of band stations, consider to install a filter ahead of the preamplifier. There are inexpensive and effective filters available for weather satellite, maritime, and ADS-B spectrum bands.
How do you know if your receiver is sensitive enough, after installing a better antenna and preamplifier? You should be able to hear the background noise from your surroundings: automobile or truck spark plug noise, lightning from nearby thunderstorms, powerline noise (yes - at VHF and UHF), along with things like navigation aids, boats on the surface, and distant weather stations enhanced by tropospheric bending on good days or evenings. When you disconnect the antenna from your RTL-SDR, the background noises should disappear. When you reconnect the antenna, those noises should return.
One important adjustment should be accomplished after you change antennas and / or add a preamplifier: adjust your RTL-SDR gain as necessary to receive weak signals, but not higher than necessary.
If you consider the above information and make improvements to your receiving station, you can rest assured that your rig is "sensitive enough." To pick up whatever is in your area.