what is the easiest trick to check whether a transistor is used as a switch or current amplifier?
If you see a circuit like this:







simulate this circuit &ndash; Schematic created using CircuitLab

It is being used as a switch.

A linear amplifier would look more like this







simulate this circuit

The specific resistor values will vary from circuit to circuit. I used the common emitter topology as it is very commonly used.

There are numerous resources for linear amplifiers, and you can read about the biasing requirements for this one at Wikipedia