Yes, Jomtien Immigration always looks in your passport to see if a TM30 slip has been stapled to your passport.
I never knew that a TM30 could be done online, so I'm wondering how this will go for those doing it online as you have no proof in your passport that it has been done. No TM30 in passport, no extension can be done.
If you do a TM30 online, do you just upload the rental contract and that's it? Does anyone actually checks your rental contract to see it's correct and valid? In person they always look through it to make sure everything is in the rental contract and it's valid.