There's no special thing about downloading video's compared with other downloads. So any INTERNET SPEED TEST (Google that) will do. it's being said by some that our own CNET speed test isn't so accurate, so better try a few, a few others maybe.
The limiting factor is either the speed your ISP offers (that's what you pay for, say 8Mbit = 1 Mbyte per second for ADSL or 120 Mbit = 15 MByte per second for the cable, even higher for glass fiber) or the speed your ISP receives the data via the Internet from whoever that sends it.
You can't influence the second. You might be able to influence the first by paying more or switching providers.
Finally, a wireless G-router is 54 Mbit/second, a wireless N-router more than doubles that. A wired connection is either 100 Mbit/second or 1000 Mbit/second (socalled Gigabit Ethernet), depending on the hardware you use on both sides.
What feature(s) in a computer should I include to maximize speeds at which videos can be downloaded? Also, how can I test the speeds of my isp, wireless router, and computer?