What is the best way to test effort during psychometric assessment of a person with a mild intellectual disability?

Good luck!

Thanks for your detailed answer.

The Test of Memory Malingering (TOMM) might be worth checking out. It's a 50-question verbal recognition test that's been shown to have sensitivity to malingering but robust enough to use with cognitive impairment (and as I understanding it, intellectual disability population). Have a look and see what you think.