how to check the amount of RAM you are using
The question is not so simple to answer.
The task manager will tell you how much fixed and kernel ram you are using.
cached memory will tell you how much data has been loaded into ram, available for use, but it does not say how much of it is actively being used.
The resource monitor can give you more detail by application.
Look at the working set numbers to see how much ram each application is using most of the time.
If you think you might be short on ram, look at the hard fault rate per second. If, for example, you get 5 page faults per second, and each one takes 20ms to resolve, then the task is waiting 10%(5x.020) of the time.
Ram is currently cheap, and there is no performance negative to having an excessive amount.