If you hunt down the specifications, there are variations between types, this covers different colours and brightnesses, and the current needed to get that brightness, but generally, a raw LED will survive having its ballast resistor calculated as if it was going to be a 2 volt device. If an LED is sold as a "5 volt" device, it should be safe to assume that it is a 2 volt LED with a built in resistor, or an equivalent bit of circuit to restrict the current to a safe value. Testing it with an extra resistor should do no harm, and could save a minor tragedy.
There are plenty of applications about, mostly small torches, that rely on the internal resistance of the battery to provide the current limiting, and for different colour LEDs you can expect different currents for the same brightness.