You cannot see (with the eyes) oxygen and nitrogen, methane or hydrogen, argon, carbon dioxide, helium, neon etc.
But you can see nitrogen oxides as products of a reaction between nitric acid and some metals or oxides.
You can see some halogen gases, such as Chlorine and Bromine and Iodine because they are coloured. Most other gases are colourless and cannot be seen.
By tinting them.
No, not usually with the naked eye.
The sun
Some gases have distinctive smells. Others may be tested for by various means (like testing for hydrogen with a pop test).
You would see aliens gases because Saturn is a gas planet like Uranus.
yes
you can't see gases and it fills the room/container.
Fart gases are mostly made up of odorless gases like carbon dioxide and nitrogen, which our eyes cannot see. The only gases produced in farts that are visible are methane and sulfur compounds, but their concentration is too low to be seen with the naked eye.
Yes, but only if you have an electron microscope
The moon does not produce its own light; instead, it reflects sunlight. The light we see from the moon comes from the sun illuminating its surface, not from any burning gases. Therefore, the moon appears bright in the night sky due to this reflection, rather than any internal combustion or burning gases.
You see million years ago there was a HUGE clump of gases which they call "nebulae" well the gases formed together and then came are star the Sun.