If they're really pants at hacking, you might find there's an app in the running apps list, but that could be hard to spot if you're not too familiar with what Android ought to run .. and would be a bit too obvious for any proficient crim.
To remove such programs you could try scanning your phone with a variety of the Anti-Virus packages - I say a variety because any single AV generally only picks up around 60% of threats but they're not usually the same 60%.
Not sure how effective AV would be, though. A factory reset (after backing up) is probably the safest option .. though, if the hacker's really good ..