Of course, you need to know their marking numbers and then look for the datasheets from the manufacturer's website. But, it is not easily as most of the small round LEDs are supplied with marking number on their boxes only, not the parts themselves.
However, it is lucky for us that most of the 3-mm and 5-mm diameter round LEDs and SMD LEDs have the maximum forward currents of about 20 to 30mA. So for safety reason, do not use any unknown LED with the current that exceeds 20mA.
Nowadays, there are types of the high efficient LEDs that called bright LEDs or super bright LEDs which beam very strongly with low forward current, about 3 to 5mA only, while they still can stand for 20mA maximum. For the old types of LEDs, you have to feed them with the current up to 10, or 15, even 20mA to get enough of light.
Hope that clear enough for you.
nguyennam