Question about surprise

[DeathBecomesus]

I will soon be runner the beginner box
I have experience (limited and many years ago) as player but new to GM role

My question

Big Spider encounter
Surprise
How to determine if party members are surprised or not?
I did not seem to read it, when I read through that section
Perhaps I missed it?

[glio]

It says on page 9, the Spider's Lair page, next to "CREATURE:"