if you use PDDW.com, you can target expiring names that have hardlinks and listings/traffic. sometimes the traffic is steady for years and sometimes it tapers off though.
TDNAM shows estimated traffic on the domains they sell. Use this as a guide but be very careful as it cannot be relied upon. You could also use keyword popularity search tools to search for the term of the domain name such as wordtracker or keyworddiscovery.
It depends. You cannot be sure that you will get the traffic after expiration. Sometimes the traffic are steady and on the other hand, it breaks as soon as the domain expires!