How can I know the actual download speed my AppleTV is getting

I could be wrong, but I think it has more to do with your internet speed (modem + router) than with the AppleTV. I think it can handle any load (the biggest load being streaming HD content), it's just a question of how fast your internet connection provided by your ISP can download it; streaming it from the router to AppleTV should have little to nothing to do with speed.

If you have problems on your LAN they are often a result of interference. Interference can be caused by other networks in the neighbourhood or from household electrical items.

You can download and install iStumbler (NetStumbler for windows users) to help you see which channels are used by neighbouring networks so that you can avoid them, but iStumbler will not see household items.

Refer to your router manual for instructions on changing your wifi channel or adjusting your multicast rate.

There are other types of problems that can affect networks, but this is by far the most common, hence worth mentioning first. Networks that have inherent issues can be seen to work differently with different versions of the same software. You might also try moving the Apple TV away from other electrical equipment.

You could calculate it from the time it takes to completely download a file of a known size.

It's not unusual for different hardware to perform differently, each will have a different type and size of antenna.

If I understand your question right, you could run the Apple Network test while running a speedtest on your network. I did this once and the Apple TV was taking my network from 11-12 Mbps to around 1-2 while running Airplay and Hulu.

If you have a Netflix account, there is a special video you can watch that will display your download speed. It works on any device.

The title is 'Example Short 23.976' Link http://www.netflix.com/WiPlayer?movieid=70136810