A phone charger is supposed not to be serviceable (you must not replace components). As this is a safety device, and even a bad solder joint can cause a serious hazard (fire, shock, phone blown).

The only way to find out the value of this transistor is by using a multimeter with diode tester function and testing the transistor as if it was a circuit of two diodes (that is what a transistor is), see here: Testing semiconductors with analog and digital multimeters. , also Transistor.
If the transistor has blown this will be impossible.
Try also going to a spare component shop with the transistor and circuit, this is what we all do when we do not have the schematics and value.

Being this a $8 charger, better buying a replacement:

Sagem my202C European Mains Charger EU 2-Pin

Sagem my202c Chargers uk 3 pin plug