Ohm's Law.

Ohm's Law defines the relationships between power (P), voltage (E), current (I) and resistance (R), which is measured in ohms. This law is defined as V= I·R and explains how to calculate the current:

In this example, there are 20V and 2000 ohm in the resistors in series. The current it's calculate like the 20V devided by the resistence, (1000 kiloohms in one, and 2000 kiloohms in other). So the current is 0,0066 Amperes.

The current can be measured also by the multimeter, mentioned above.