About a year ago i built an IR controlled remote control car. In order to initially decode the remote signals, i used an oscilloscope that was hooked up to an IR receiever circuit. i setup the oscilloscope in single shot mode, so once the signal on the IR receiver changed, it would take a snapshot of XX number of milliseconds. From that i was able to see things such as the start bits and the data of the signal, and essentially decode it.

Be sure to make sure that there is as little IR noise as possible when doing your research or testing, since that will affect your signal. If you don't have access to an oscilloscope, some of the remote manufacturers have the signals available on their websites.

as far as decoding it using an 8051, it looks like Ijaz's post pretty much covers the basics of taking in the data, and comparing that to specified values.