The easiest way would be if you just installed an app and this started happening. I would uninstall that app and see what happens. Updates to apps and widgets can do the same thing.
You want to do this after it has discharged for a while. Go to settings>system (tab at the top)>battery. There you should see a big battery. hopefully battery monitoring is on. If it isn't, turn it on and this method won't work for now. If it is on select "battery use". It should bring up a page with a graph at the top and a list of apps with some percentages. Press the menu button and select "unplugged usage". The screen should look similar but should show you what app is crashing the party. Android system, Android OS, cell standby, display may all be near the top and this is normal. I would suspect you have an app that will have a large percentage (20%+, maybe even 40-50 as fast as you are draining)
hope this helps