Hard to give a definitive answer.
- Check Login Items (in System Preferences) to see if there are any apps starting up that you don't recognise.
- Try "launchctl list" in the Terminal, and see if there's anything there that shouldn't be. (Not always easy to tell).
- Install Little Snitch, or something like that. A keylogger is little use if the person who installed it can't get at the recorded data.