How can I tell if my Molly fish is pregnant

How do I tell if my fish is pregnant and if so how long tell she has them should I get a box for her or just buy another aquarium any advice would be helpful I have never owned fish before

You can assume that all livebearer females are pregnant, all the time.

You don't need to do anything except;

make sure you have good water, so you need test kits if you don't have them already, and especially if your tank is new

provide hiding places for the fry. Live plants are best; pond plants are cheap and you can let them float. Livebearer fry like to stay near the surface, so you need tall or floating plants.

It's best for the female and the fry to stay in the main tank.