What would you observe or measure if an anhydrous was left on the lab bench for a few days

AP Chemistry lab report.

You would be able to measure an increase in mass, as the material took up moisture.

In some cases, there may be a change in color. For instance, CuSO4 is sort of white, but as the pentahydrate, deep blue.

If deliquescent, you will see beads of moisture, or even a puddle. NaOH pellets will become shiny with the water on the surface upon exposure to the air.

The sample will slowly gain mass, as it absorbs moisture from the ambient air. That's why some samples are stored in a desiccating jar, rather than out in the open.