John Jonas advises employers who suspect an employee is working multiple jobs to ask the employee directly and discuss their circumstances. He recommends understanding the employee's financial situation (e.g., whether they need money to survive or simply want extra income) and working with them compassionately to find a solution; he reports positive outcomes when handled openly.
Chapters
If you suspect that you have someone working for you either part time or full time, and you suspect that they have taken on another job or they're taking on multiple other jobs, ask, ask them about it. I've had a couple of situations where I suspected it, but I didn't know and I said something and we worked it out. It was exciting for them that I wasn't super mad at them and that I was willing to keep them. And they've been really, really great since that happened. So asking. And then when you ask, it's acceptable for you to try and take care of people and like talk through their financial situation with them and see why are they trying to do, what's their situation, Are they single and they just have more time and they just want to make more money, they want to really want to work really hard, or do they have a wife and two kids and they're just not making enough money and they need more money to survive? You know, like those are different situations, but once you know the situation, you can work through it with them, hopefully.