DC Adapter - AC Potential to Earth
Capacitance between live AC terminals and DC output wires is not zero meaning that if you measure the AC content on the DC output with respect to earth you can easily measure tens of volts AC. For a decent power supply this might cause a little tingle if you touched those outputs while being earthed. This is not unusual.

So where does the capacitance come from - the internal switching transformer has capacitance between primary and secondary and that is one route. Another route is capacitors placed between live and DC side to reduce radiated and conducted emissions due to the high frequency switching.

So you have capacitance and you'll measure a few volts to a few tens of volts.


  When trying to source a new adapter is this something I can find in
  the datasheet? What controls this in an isolated switching power
  adapter?


Probably not.