The real question isn't finding absolute certainty, because that doesn't exist. What you need to do is look at how you feel day-to-day with him and whether you're moving in the same direction. Do you work through conflicts together? Can you be yourself without forcing it? When you imagine yourself 5-10 years from now, do you see him beside you, or is it fuzzy? Those little things tell you way more than occasional doubts.
Doubts are totally normal, honestly - they're not automatically a bad sign. A lot of people have them before a serious commitment because let's be real, it's a huge decision. But there's a difference between "I have normal fears about marriage" and "I'm not sure I'm with the right person". If 99% of the time you feel good, you get along, you make each other laugh... that 1% of doubt might just be your brain liking to overthink things. But if most of the time you have the feeling something's missing or it weighs on you, then you need to listen to that.
And honestly, nobody can tell you "he's the right one", not even you right now. What matters is the decision you make with him, not finding some magical certainty. If you really want to move forward, talk about it together. Not to ask him "am I the right one?" but to talk about what you actually want, your respective fears, how you see things going. Often when you get it all out there you can see things more clearly.