Minimum proof for photocatalytic CO2 reduction?

Photocatalytic CO2 reduction is beautiful, but it is also where false positives can sneak in quietly. I would like this thread to collect practical checks that catch bad signals early.

The older review is a helpful map of the field: doi:10.1002/anie.201207199. The false-positive paper is the one I keep returning to: doi:10.1021/acsenergylett.2c00427. For isotope tracing, Wang et al. is a good anchor: doi:10.1038/s41467-023-38052-0.

If a paper reports CH4, CO or methanol from light plus CO2, what controls do you want before believing the carbon came from CO2?