Almost all species of pet fish can lay eggs, however, it is not always an easy task to tell when they are carrying them. The most common way to tell if one of your pet fish is carrying eggs is through the bulge. However, that bulge can be extremely difficult to spot, as it blends in with the natural shape of the fish. Furthermore, it is rare that you will see fish, especially small species, stay still long enough for you to spot the bulge.
The most common pet fish that actually give birth to live fish are Guppies, Mollies, Platies and Swordtails. Whether you have fish that give birth to live fish or ones that lay eggs, it is very difficult to raise them in your primary tank. That is simply because larger fish may see them as food; even vegetarian fish. Furthermore, the larger fish in your tank are prone to chasing and terrorizing the smaller baby fish. That can lead to exhaustion or injuries, and even death.
The best way to combat this problem is to have only one sex of fish in one tank, which can be difficult, or to have a separate tank for after birth.