If you multimeter has a transistor gain function you can often tell the difference between collector and emitter. In my experience when the emitter is used as the emitter you get a higher gain than when the collector and emitter are swapped.

There have been very few places I would use a transistor that is largely unknown. When I receive transistors that are clearly not as advertised they usually go straight into the trash. Transistors are cheap these days but my time has become more precious lately.

 

in my case: I insert multimeter: red(+) in base [mid pin] - and - black(-) first or third pin, and contacts[show in multimeter],...

NPN 2N3904 - E-B-C = in this case, E or C can be used interchangeably??? If No, how to distinguish between???

 

Read post 21.

But you are wasting your time. If the part is a 2N3904 there is a 99.99% chance it will be EBC when viewed from the flat side with the pins at the bottom.

Bob

 

NPN 2N3904 - E-B-C = in this case, E or C can be used interchangeably??? If No, how to distinguish between???

Beta will be very low if you have the emitter and collector swapped.

 

BJT practice in breadboard

Bipolar transistors have three(3) poles, like The 2N3904 NPN
Middle pole is base?
The left/right poles are chosen arbitrarily,...? If No, how choose the collector?

Ground (-) what should be, and what (+)...?

This has a fantastic introduction to BJTs, their configurations, how to control them, walkthroughs, exercises, etc:

Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3