How to check if there is electric current on material surface?
The surface has a common mode potential from the AC line filter and the SMPS transformer which both contribute to surface voltage relative to earth ground. The current is not through the surface until you are connected between earth ground and the metal case. THese currents are minimal yet exist for EMC reasons and for safety reasons, left floating in case of ground faults and you get between them.

There are two paths that connect you to this surface potential to earth ground.

1) Y caps on CM choke to earth pin in charger are limited by safety design to 250uA at line f.

2) transformer is floating between AC and DC out yet there is still leakage from stray capacitance primary to secondary now at SMPS rate because the wiring is not balanced like a CM choke and DC CM chokes are large and expensive, so they exclude them and just add a small CM choke in the form of a Ferrite torroid or clam-shell ferrite molded around the DC cable.

Hence a small skin surface area in MOhms range ( if measured on a DMM)  is high compared to the Zc(f) of your skin contact with a large dielectric behind it (you) and a large surface area ( your feet with sweaty socks and leather shoes on non-dry concrete or other hand touching earth ground )  Thus the impedance of the stray coupling capacitance and primary voltage is the path which tends to have a burning sensation more than the Y cap tingle at line f. 

Both are safety tested for Y cap leakage , but not always correctly tested with secondary earth ground currents measured.

I too have a concern with this.

Yet designers are aware this must be avoided to prevent these RF burns from switched pulses with fast risetimes and the stray series capacitance is chosen to limit this annoyance sensation which is only serious in medical applications.

If someone has specific safety standards to test unbalanced RF leakage currents via transformer coupling capacitance, you may add to this.