A leaking shock absorber is broken and need to be replaced . That they do check at the dealer or technical inspection . Another test they do is a difference between L and R shock's . If the difference to big , change it . The best test IMHO is the feeling you have yourself . If you find the car less stable , or bouncing , or something else , worse then driving it in the beginning , change them . Off course , the difference needs to be a serious change , otherwise you can change them every 20 k . But really , feel it in corners , when you feel it really going a lot more unstable or something like that , it's better to change them . When , well that's something you have to feel for yourself but 80 k can be perfectly possible .