Hyper-V Program Manager
Under Virtual Server you can configure the amount of memory that your virtual machine will have. However - when this virtual machine is actually running it will use slightly more memory than what you specified (to deal with information the Virtual Server needs to store as part of running the virtual machine). Seeing how much memory is in use by Virtual Server is not exactly intuitive. You actually do this under the Windows performance monitor. You can do this by selecting to 'Run...' off of the start menu and running 'perfmon'.
This will give you a default performance monitor set. You will need to add the performance counter for memory usage by Virtual Server. To do this - hit the 'add counter' button (a '+' button). You will now need to close this dialog and re-open it (I do not know why - but we do not appear in the counter list the first time). Now you can select to look at 'Virtual Machines' and then 'Allocated MB'. This will allow you to determine exactly how much memory is being used by your virtual machines.