We've put together a quick tutorial on one of the questions we get asked the most about MIDI files: "How do I tell the musical key of a MIDI file?". The answer is to open up the MIDI file (.mid) that you'd like to find the key note of in your DAW's Piano Roll Editor - virtually every DAW has one of these, and they work in pretty much the same fashion whether you're using Ableton Live, Logic, Cubase or any other DAW for that matter.
Step one is to identify the bass notes of the sequence in the piano roll editor. You can see in our example that we've used Logic's velocity tool to highlight the notes to easily pick them out. Once you've identified these notes you can use them as the basis of the creation of your tracks's bassline.