How do I know current across unknown load if I know voltage
One approach is to use a hall effect sensor to measure instantaneous current and a low end microcontroller to drive an output led or lcd panel.
The Allegro ACS 712 is fairly simple to use, and you can find breakout boards for it at low cost at sparkfun. 
datasheet here: http://www.allegromicro.com/~/media/Files/Datasheets/ACS712-Datasheet.ashx?la=en