RIght now? You do not. Sadly. THer are third Party tools for that, though.
SOON: Use WIndows Server 2012 oR windows 8 - RDP is significantly improoved, including the ability to sense available bandwidth and for example even play videos over RDP, preferably with a graphics card on the server that is used for encoding. The keyword is RemoteFX. Helps to run virtual machines on a server you control so you can enable it.
Sadly THat is still some months in the future, though with the release candidate out in 2 weeks time, I think I will make a test... get a new machine on the 8th that is not used for a month, only setup tests, so I will give it a try there
the biggest and easiest saving is to disable the background image and go all plain color. Not all RDP clients support this as a client setting, so do it on the server.
Re bandwidth: an easy way is to use the Task Manager and look under the networking tab. On my 1900x1200 full color desktop I'm pulling about 8k/sec.
You can cut that into 1/3 by carefully playing with the settings.