The only way to determine compression is to record a fairly long timeframe of SD programmes (say several hours) onto a DVR seeing how much HDD space is occupied and then backcalculate the bitrate. I see you have a DVR from the first post.
See the first post of the following thread under HDD Space available.
http://www.digitalhomecanada.com/forum/showthread.php?t=13405
Steps.
1. Record a long programme - say 10 hours.
2. Look at the number of "units" on the HDD (write down the value).
3. Delete the programme.
4. Look at the number of "units" on the HDD now. (write down the value).
5. Subtract one from the other to see the number of units used for that recording.
6. Based on the number of units, calculate the percentage of HDD used.
7. From the percentage, calculate the number of GB used.
8. Use the number of GB/10 hours to calculate Mb/sec (remember 8 bits/byte).
Your answer should be around 3-5 Mb/sec. See the following example which I did for 2 hours of HD. Remember, this will not be perfectly accurate since the HDD is not 100% utilized for programming, but it will allow you to compare several different channels.
http://www.digitalhomecanada.com/forum/showpost.php?p=322090&postcount=5
If you don't want to calculate the actual bitrate, you could just record two different channels for the same amount of time and see how many "units" are used for each to compare...