Weak Rear Shock?

I have a 1999 YZ400F and a 2001 XR400. I have read that the seat height on the YZ should be around 38" and the XR about 36". The YZ has a lower aftermarket seat, but it stands quite a bit lower than the XR. It seems to have good dampening ability, soaks up hits well, and I havent noticed any leaks. How do you know if your rear shock is worn out? How much sag is considered normal for this bike? I can pull up on the rear fender and it raises the height probably 2 inches or so.

Thanks

I have read about sag on this site. It seems the general consensus is 25-35mm of static sag and about 100 mm of rider sag is about right. I can touch flat footed on the bike, which is good because I trail ride. Starting that big motor on the trail without being flat footed would be difficult. It seems that sag is more of a factor with the spring adjustment than the shock itself. That takes me back to my original question..how do you know when the shock itself is worn? I will take some measurements tonight.