For this purpose we use different electrical instruments to detect the current in the circuit.Galvanometer and ammeter are some common examples of current measuring instruments.
Galvanometer is very sensitive instrument and can detect small current in a circuit.A current of few milliamperes is sufficient to cause full scale deflection in it.While making the connections polarity of the terminals of galvanometer should be taken into consideration.Generally the terminal of galvanometer with red colour shows the negative polarity while that of with black colour shows the negative polarity.An ideal galvanometer should have very small resistance to pass maximum current in the circuit.
After the suitable modification galvanometer can be converted into an ammeter.A large current of the range such as 1A or 10A can be measured by mean of ammeter.Like galvanometer,ammeter is also connected in series,so the current flowing in the circuit also passes through the ammeter.