safety of carbon fiber bonded to aluminum fork crown
Carbon fiber is a bit tougher to determine the safety of than steel or aluminum (where a thorough visual inspection normally reveals the serious flaws, along with checks of headset tightness and what not) - aside from a thorough visual inspection for cracks and damage all around, you really need a professional evaluation. Bikeradar has some tips for inspecting carbon fiber bikes - key points are bonding areas and essentially places where stress builds up and making sure the bike hasn't been crashed before. There are plenty of guides on buying used bicycles online and safety checks to perform on them. 

As for lifetime, especially in the case of a high end carbon fiber bike, the lifetime is a function of how much it has been used. A crash can do a lot more damage to a high end carbon fiber bike than to a lower end steel/aluminum bike (due to likely higher speeds and weight savings). There was a previous question here which highlighted the lifetime of carbon fiber bikes (especially forks), which you should read. In particular, the article by Zinn is useful.