The Super Saiyan and Super Saiyan 2 forms look incredibly similar in the Dragon Ball franchise, but there are subtle differences between them. Over the years, various levels of the Super Saiyan transformation have been discovered, including Super Saiyan Ultra, Super Saiyan 3, Super Saiyan Blue, and more. Most are visually unique in one way or another, but this isn't the case with Super Saiyan and Super Saiyan 2.
The Super Saiyan 2 is one of the only major Super Saiyan forms not achieved by Goku first. The form was first accessed by Gohan during the Cell Saga. During the Cell Games, Goku fought Cell for the fate of the Earth and abruptly gave up, knowing that he could never win. As a Super Saiyan, Gohan could barely hold his own against Cell, but after hearing words of encouragement from a dying Android 16, Gohan cast off his restraints and went Super Saiyan 2 for the first time. Without this upgrade, Gohan would never have been able to defeat Cell and save the planet.
How can one tell the difference between the Super Saiyan and Super Saiyan 2 forms? Since the upgrade from Super Saiyan to Super Saiyan 2 doesn't involve any noteworthy physical changes, there aren't any obvious differences, and there are times when fans may not even notice if a character has simply powered up or if they've accessed the Super Saiyan 2 transformation. The most distinguishing characteristic of this form is the blue sparks of lighting that radiate from the character's body. The aura around the character is a bit brighter as well.
There's also a slight change to the hair. With Gohan in the Cell Saga, the difference here was clear, as Super Saiyan 2 Gohan's hair stands up on end, and this makes his appearance quite unique to his Super Saiyan look. But with the other characters, the change is a bit more subtle. With Saiyans like Goku, Vegeta, Future Trunks, and Caulifla, on the other hand, their hair extends out to the sides more so than it does when in the standard Super Saiyan form. Goku, in particular, has fewer bangs as a Super Saiyan 2.
Even though the Super Saiyan 2 form can't compare with what heroes like Goku and Vegeta are able to do now, it's still relevant in the Dragon Ball franchise; it's used several times in Dragon Ball Super. The form is utilized by Goku, Gohan, Vegeta, Future Trunks, and two of the Universe 6 Saiyans, Cabba and Caulifla. Since Cabba and Caulifla are significantly weaker than Goku, Vegeta, and Gohan, going Super Saiyan 2 was a considered a big step for the two characters, especially Caulifla. Caulifla went Super Saiyan 2 prior to the Tournament of Power but couldn't reach that level a second time until she had her limits pushed by Goku. And as a Super Saiyan 2, Kefla nearly defeated Goku, who had to use Ultra Instinct to take her down. So even though the Super Saiyan 2 form isn't as powerful as some of the other forms, it still has its place in the world of Dragon Ball and can be distinguished from the original Super Saiyan transformation.