How do I check whether a file is available to download?

The following code generates a link to download the HD file from a site every day, but it doesn't know how to determine whether the link is valid or not. How do I check the validity of the link?

set z to ""
set d to tid(tid(short date string of (current date), "/"), "")
set w to weekday of (current date)
set r to 10

if d > 20000000 then
    set Prefix to d as text
else
    set Prefix to (d + 20000000) as text
end if

on tid(input, delim)
    set {oldTID, my text item delimiters} to {my text item delimiters, delim}
    if class of input is list then
        set output to input as text
    else
        set output to text items of input
    end if
    set my text item delimiters to oldTID
    return output
end tid

set x to 1
set y to ((characters 1 thru 4 of Prefix) as string)

repeat with i from 1 to r

    if i < 10 then
        set x to "0" & i
    else
        set x to i
    end if

    set c to "http://streaming.hkjc.edgesuite.net/hdflash/racingfocus/" & y & "/" & Prefix & "/" & x & "/" & "chi/racingfocus_" & Prefix & "_" & x & "" & "_chi_2500kbps.mp4"

    set z to z & c & "" & return & return
    set i to i + 1

end repeat

display dialog z

I usually check the HTTP Code and then run from there. Since this is tagged AppleScript here is an example handler I wrote:

use AppleScript version "2.4" -- Yosemite (10.10) or later
use scripting additions

set foundMP4 to my checkStream(c)

on checkStream(passedStream)
    try
        set serverCommand to "curl -o /dev/null -Iw '%{http_code}' " & passedStream
        set serverResult to do shell script serverCommand
        if serverResult does not start with "2" then
            return false
        else
            return true
        end if
    on error serverResult
        display dialog "Ran into issue checking server status, with error: " & serverResult with title "Server Check"
    end try
end checkStream

After you check the HTTP code you could write for the script to download the file.