Whilst a lot of tab does have the H and P letters to indicate a hammer-on or pull-off, I have come across tabs which just show the curved line.
The way to tell, in this case is, if the second note is a higher pitch then it’s a hammer-on; if it’s a lower pitch then it’s a pull-off.
And, of course, if it’s the same note, then it’s a sustained note.
Cheers,
Keith