Use "supposed to be" when talking about something that is generally thought to be true.
Examples:
▶ Paris is supposed to be the most romantic city in the world.
▶ Your boyfriend took you to dinner at Masa? That's supposed to be the most expensive restaurant in the city!

Use supposed to + infinitive to:
==> Say what should or should not be done because of rules, common practices, or customs
Examples:
▶ Before ordering supplies, you 're supposed to get your boss's approval.
▶ You're not supposed to smoke inside this restaurant.

==> Express sarcasm and/or anger, in place of should, can, or going to
Examples:
▶ Who was the nineteenth president of the United States? How am I supposed to know?(= I don't know! How would I know?)
▶ Your music is so loud, how am I supposed to get any work done? (= How can I get any work done with that loud music? I can't!)
▶ You invited your entire office over for dinner? Who's supposed to do all the cooking?(= I don't feel like cooking for all those people! Who's going to do all the cooking?)

==> Express something that was planned or intended, but did not happen (in this case, use "was/were supposed to")
Examples:
▶ Luke was supposed to start college in the fall, but then he decided to travel around the world instead.
▶ We're lost! We were supposed to take a right onto Danbury Road.
▶ We were supposed to go to a holiday concert last night, but it was snowing too hard.