A year, sometimes month too, birthday may be on the label somewhere.
In terms of how much use, a SMART reader can give u a fair idea, u can run them from DOS. google is ur friend.
I'd mention what Jim's saying too, if ur m8 has a SATA-1 platform, first of all the drive will need a jumper - some don't offer this - AND if he was trying to install windows on it, it won't see anything partitioned above 137g. This is all if the build is an old one.
Strangely enough, u can partition it to 137g, install windows, then use the rest.