Any ideas on how to detect being the subject of such a simulation without prior knowledge that the upload would happen, or that uploading even exists?

I assume "without prior knowledge" because from the perspective of the administrators of such infrastructure, it would be beneficial if the simulated subjects did not know that they're being simulated:

This would increase their compliance greatly.

Making them do the desired work would then instead be conducted by nudging their path of life towards the goal of their simulation.