It's easy to tell of there blown. Try wiggling the strut shaft, if theres play in it chances are it's shot. There doesn't need to be any oil leaking from it for it to be blown either, I've blown two and neither leaked any oil. I think with Nitrogen charged struts like the OME's, the oil is sepereated from the nitrogen by a bladder. OME's are more likely to blow out from topping out ( FULL EXTENSION UNDER RAPID AND REPEATED CONDITIONS) than bottoming out. Since I've put the limiting straps on mine I've had no strut issues, and just wheeled Moab and two National parks, plus a 5000 mile round trip. Limiting straps are the way to go to save OME's, imo.