You can do it either by connecting a multimeter to resistance measuring mode with a scale of 1000 ohm (since the coil resistance normally ranges between 50 ohm and 1000 ohm) or by using a battery. Let’s start by determining the relay coil pins. I have attached the datasheet of the relay for more information. L: is the 'Coil Sensitivity' which is 0.36W 05VDC: Also known as 'Nominal Coil Voltage' or 'Relay Activation Voltage', it is the voltage necessary for the coil to activate the relay. It starts from and goes down till Finally the bottom bit says: SRD-05VDC-SL-C SRD: is the model of relay.

Then we see the ' Current and Voltage Rating': it is the maximum current and/or voltage that can be passed through the switch.

Now, if we look at the top of the relay the first thing we see is SONGLE, it is the name of the manufacturer. The closing and releasing of the contacts results in powering on and off of the circuits. When the coil is de-energized the electromagnetic force disappears and the armature moves back to the original position turning on the NC contact. When the electromagnet is energized the NO is the one which is on and NC is the one which is off. When current flows through the coil of the relay, a magnetic field is created that causes a ferrous armature to move, either making or breaking an electrical connection. Middle one is COM (common) and the rest of the two are called NO (Normally Open) and NC (Normally Close).

It is very popular relay among Arduino and DIY electronics hobbyists. The relay I am looking at is a SRD-05VDC-SL-C.

Arduino relay module