I've bought a JDM 1990 turbo SW20 about a month back and the history is very vague.  It runs pretty well and I was planning on replacing the timing belt until I saw the feedback on the complexity of the job and all the other work that is suggested while changing the belt.  Inspection of the belt does not look too complex but I wondered how good a guide this is to deciding on how long the belt might have been in?  Is there anywhere on the engine where people tend to make a note as to mileage of last belt change?  Thanks

when you buy a new genuine timing belt there is a sticker that comes with it much like the service sticker shops put on your windscreen to tell you when next service is due.
 
this sticker should be attached to the top of the timing belt cover with the date and km's recorded of when the install was performed.