If it's on the Earth, you can tell by the effects of weathering. The same on other planets, like Mars, that has some weathering as well. On bodies such as the Moon and others that are airless, it can be determined to some degree by destruction of parts of the crater wall (as that would have have to occured later) and by the amount of dust inside if you have the means of measuring it. I'm sure there are also others but that's what came to mind on the spur of the moment.
Visually, older craters will have impact craters inside of them. There are ways of dating when the rocks in a crater were last melted and re-solidified if you have the rocks.
