What potential employee 'hacks' can we look out for and how to pick up on these?

Wiggling the mouse every minute or so will keep the idle time down. But that's a job in itself to keep up. Understanding the job, its duties, and a good estimate of how long tasks and operations take on a daily basis will give you better trust in the final numbers. I use this app for my developers. In the first pay period, one developer's stats came back with 40% idle time. I found out that he uses a PC and Mac to complete certain tasks. So if the clock is started on one machine, and he's performing operations on another, it will make the numbers look skewed.