- Take off the little black plastic caps on top of the strut shafts
- Try rotating the nut with a wrench.
- If it turns easily, without spring-y feel, the strut mount is busted.
It can still be partially attached and not turn all the way, but if the gap between the rubber ring and the strut tower is either very small or very large it's a telltale sign.
There can be a number of other reasons for the clunks, and it's usually best to do a full rebuild of the front suspension if you tackle it yourself and if you got any kind of mileage on the car. Something else will give up pretty soon anyway. Replace struts, strut mounts and bearings, lower control arms (or just bushings if you have mad skills), ball joints, sway bar end links. Also, change XC90 axle bolts at the same time.
Internet-diagnosis of clunks and squeaks is hard, so take it to a mechanic it you just want to replace what's broken now.