The easiest way to do it is to use cookies. Put a cookie on the user that tells you if he has been on the site. However, cookies are not reliable, since your user might have turned them off, or it may not be the first time he is visiting, and you would have to do a lot of housekeeping. On the other hand, you could do it with ASP, assuming you run on a windows server. The trick here would be that the part of the Javascript code that passes values to Flash, would have to be written from ASP, uponpage execution. First, you would check the HTTP_REFERER variable,and if they are coming from within your side, you would write a certain value, Eg:
<%
if instr(HTTP_REFERER, "yourdomain.com")>0 then
FlashStartFrame = 10
else
FlashStartFrame = 1
end if %>
Then you make your flash movie retrieve this value from the page every time it loads.
I hope that helps.
Good Luck!