Are benchtop lab power supplies usually isolated?
Depends on what you mean by isolated.
Isolated from input: yes, lab supplies with human-contactable outputs will be galvanically isolated from the input supply (usually AC) via a transformer. This protects from shock hazard.
Isolated from earth: depends, but generally yes for &quot;true&quot; lab supplies (and no for repurposed things like PC ATX supplies). Check the datasheet if in doubt.

This type of supply has a metal bracket which allows you to earth the negative rail. If you disconnect the bracket, it's floating. You can usually connect these types of supplies in series to create a higher voltage rail (within some limits).
If there is not a discrete earthing terminal like the picture shows above, the best way to know for sure is to measure between earth and the outputs with a DMM.