How to tell a nebula from a galaxy? Not all galaxies are spiral in shape[1][2], some nebulae are huge[3] and nebula are the nursery of stars[4]. How to tell them apart?

Note

I have already compare the contents for galaxy and a nebula, they are very similar as both have stars, planets and dusts.
Galaxies are a large organised collection of stars (&amp; nebulae) outside of our own galaxy, very distant. They shine by their own light.

Nebulae are clouds of gas &amp; debris from, usually, a stellar explosion (Nova, supernova, etc), within our own galaxy, lit up by nearby stars (maybe internal ones) - but the gas is not glowing with it's own light, generally.

So a galaxy is going to be a bright clump of glowing stars, regular in shape - be that elliptical, barred or other, generally they will appear symmetrical &amp; regular. A nebula will be much more amorphous &amp; patchy, possibly with shapes from the clouds of gas (c.f. Horsehead, Orion), and you will be able to see individual stars in it - unlike a galaxy where they are too tightly packed &amp; distant to make out individual stars.