Depends on the fish. Some lay eggs immediately during fertilization, so they aren't ever "pregnant". On livebearers like guppies, a swollen abdomen or enlarged gravity spot are the most common signs.
No, unless its of the same breed
When you hold the fish it will say what kind its status(which will say pregnant) and age
the fish will have dots
When it is floating not swimming.
Simple observation. You will either see them fighting or see the damage on the fish that indicates they have been fighting.
Exactly as you spelled it in your question; a Betta fish is also known as a Siamese Fighting Fish.it is spelled "Betta", or its other name, "Siamese Fighting Fish." I know alot about Betta's. :D
No, fighting fish (Bettas) are only interested in fighting with other fighting fish.
The only time a male and female betta or fighting fish should be together is when they breed. Otherwise it's better to separate them before one or the other dies.
Silver dollars do not get pregnant. They are egg layers.
Of course they can. How else would we have fighting fish.
I do not know. I would highly consider asking you local fish veterinarian.
slap them with a fish, its the best way to be sure :)