This would depend on the way your website was made. If it uses a database such as MySQL i would hope that the pages have a 'created' date or 'last updated' but it depends who built the website as they might not have done that. In theory assuming you have one of these columns you could do a query to bring back a list of everything that is older than a certain date.
If they are individual html page (i hope not as that would be very difficult to manage) then you would have to look at last modified dates on the files themselves.
What language are the pages using eg PHP, ASP, JSP it will normally be at the URL (sometimes its hidden)
If you can give a bit more info on this then hopefully we can help you further.