Well Ting Ting ,  "Polarity" => direction,or being either negative or positive (poles)

In a BJT - flow of current (because it is such a device) is due to 2 charges/poles viz electrons and holes.

While FET - flow of current is due to formation of a channel of single charge/pole say n-channel (electrons) or p-channel (holes)

How do you say?  it is either - based on its construction and characterization or read the datasheet (from the manufacturer) , use multi-meter ( resistance measure).

Normally BJT s are characterized by majority carriers , i.e NPN - electron while PNP holes , same with FET

Well Ting Ting ,  "Polarity" => direction,or being either negative or positive (poles)

In a BJT - flow of current (because it is such a device) is due to 2 charges/poles viz electrons and holes.

While FET - flow of current is due to formation of a channel of single charge/pole say n-channel (electrons) or p-channel (holes)

How do you say?  it is either - based on its construction and characterization or read the datasheet (from the manufacturer) , use multi-meter ( resistance measure).

Normally BJT s are characterized by majority carriers , i.e NPN - electron while PNP holes , same with FET

BJT is  current operated whereas  FET is voltage controlled device. BJT (bipolar junction transister) is bipolar ( input- forward biased, output- reverse biased). FET(field effect transister) is unipolar (input - forward biased). 

BJTs and FETs can be identified from the values printed on the body and refereing to the datasheet or checking the input - output resistance using a multi-meter. 

adding to what has been said by the colleagues, bipolar means that the conduction in the device is affected by the transport of the two types of charge carriers in a semiconductor, the electrons and holes that is the majority and the minority. While uni polar means that the current conduction is dominated by one charge carriers either electrons or holes. Therefore the devices are classified according to this property to bipolar devices such as the pn junction diode and the bipoolar transistor and unipolar transistors such as the the Schottky diode and the field effect transistors.

In order to make the things more clear, assume that we have  a pn diode which is forward biased, then the total current is made of holes injected from the p-side to n-side across the junction and a back injection of the electrons from the n-side to the p-side.

In nMOS transistor for example the source and drain are heavily doped n-type and the channel is u-type. So the drain current is affected by the movement of the electrons in the channel. The holes are very very samll compared to the electrons and plays a negligible role in the conduction. For example if the electron concentration is 10^16/cm^3 the hole concentration will be  10^4/cm^3 for silicon. So, the ratio of the electron current to the hole current is 10^12.

Wish you success

BJT is voltage controlled current source. The base current is just a "side effect" -  see some comments of Barrie Gilbert.

https://www.researchgate.net/post/How_should_one_present_the_essential_and_fundamental_properties_of_a_bipolar_junction_transistor