There’s not an easy way to just look at a tab you’ve never heard and know the melody for sure. It’s helpful to listen to the song to get a sense of the melody and then look at the tab.
A lot of times the melody will be the fretted notes and the open strings will be an extra part of the rolls but not always.
For this particular song the melody is mainly on the 1st and 2nd strings for measures 1-14 and then goes down to the 3rd and 4th strings for 15-21.
If you have any other questions let me know. Keep picking.