While there is no direct way to tell that your tetra fish is pregnant, there are many clues and signs that can help you find out if your fish is about to spawn. From the size of the fish to multiple nests built around the tank, you will observe different characteristics during the gestation period of the tetra fish that can help you determine whether it is pregnant.
Note the size of your fish. Have you noticed that your tetra fish has gained mass, especially around the abdomen and tail? A female beta fish's abdomen will grow before it spawns. There will also be a dark spot on the underbelly of the fish near the tail. The dark spot usually shows right before the fish is due to spawn.
Observe the actions of the female beta and the male beta in the tank. Female tetra fish that are about to spawn will usually be chased by males. The chasing encourages the female fish to spawn. This can often be accompanied by an increased aggression from the male fish toward the female fish.
Look for bubble nests throughout the tank. Tetra fish build these nests far in advance of spawning. The nests are simply a huge bubble mass that is formed by saliva. There are often items in the tank included in the bubble mass, including plant material and small stones.