Get the charging rate without external hardware (e.g. multimeter.)
You can try Ampere (I didn't test it), which should show you the current (dis)charge rate:

 

Quoted from a rating:


  I appreciate that this app displays which of my available charging devices delivers the most milliamps. It's useful because I know which charger to swap out for maximum power. Also, it may indicate if a known charger may be under performing based on available specs.