How to know if a web page/file is downloaded completely

Hello,
Hope this is the right group for this question. As my earlier posts,
we developed a packet sniffer. We are interested in listening to http
packets and extract its header because we need specific values from
it. Just need some advice and clarifications for doing this. How do we
know that the web page or a file from the web server was successfully
downloaded? Is it by just looking for the 200 OK status? If that is
the case, how about web pages that are big, it will have several
packets with 200 OKs. What packet should we be looking at? Another
thing is if we use the content-length header field to check the file
size. How do we actually compute for that(message body) since the
packet still contains the headers? Thanks in advance!