How do I know if I need to use a resistor in a circuit?
How can I tell if I need to use a resistor in a circuit?


You can tell because the circuit includes a resistor. 

If a circuit needs a resistor the circuit diagram will include a resistor. If it doesn't, then it won't.

The circuit for connecting an LED to an Arduino consists of an Arduino, an LED and a resistor.  Because of that you know it needs a resistor. Because it says it needs a resistor.

A resistor drops a certain voltage across it when a specific current flows through it (Read up on Ohm's Law...). If you need to drop a specific voltage with a specific known current, or if you need to cause a specific current to flow with a fixed voltage, then you use a resistor for that job.

You don't just throw resistors willy-nilly into every circuit you make simply because you see one being used with an LED.

If a device says it runs off 5V and uses 300mA current then you know that you need to hook it up to a power source that is 5V. You also know that the module will use up to 300mA of current from that source, so your source has to be capable of providing at least that much current.