If I understand incorrectly, and you are refering to the marble color pattern, and not the marble gene... disgregard the following.
If you are refering to the marble gene (afraid he'll change colors from this current color) there is no way to tell if he has that gene until he gets older.
My husband's betta (Sushi, pictured in my siggy with 2 photos) was a 100% royal blue crowntail when we got him. Markings, color spots, and other odd colorations both are not always initially present on a betta with the marble gene. I didn't have any hint of marble anything. When he started changing colors we thought he was ill. You can see his drastic color change that happened within many months, and colors can change again and again and again.
Also, marble coloring and the marble gene do not necessarily go hand in hand. If their colors never changed other than to darken, they were marble colored but without the marble gene.
Any betta of any color can potentially carry the marble gene, but it's often impossible to know they have it until they are older.
Though... I agree with you... I hope he doesn't change, he's gorgeous! You got a beauty :D