After you have completed your circuit, you can check its properties by adding a meter. The Ammeter is just another element, like a piece of wire or a resistor: you drag it onto the grid and make it a portion of the circuit. When current starts to flow through it, click the Ammeter tab at the bottom of the screen to show the current (in Amps) as a function of time. For ordinary DC circuits with a constant current, the graph will show a horizontal line. There is only a single ammeter, so you may have to move it from place to place to sample currents in a circuit with multiple loops. Currents running from the negative end to the positive end of the ammeter are considered positive; currents running the other way are negative.