Suckers are easy to miss and grow fast. What I'm sensing is that you think you need to know because the sucker is somehow different than the stalk. It's not; it's just another stalk, so it doesn't matter which, if either, you remove.
I think that would depend on what the two stalks look like above what's shown in the photo. If you're determined to remove one, choose the smallest. Otoh, if they both have developing fruit on them, why not leave them them both? More tomatoes!
Thanks for the
Generous of you.
