Quae ubi intravere portas, non quidem fuit tumultus ille nec pavor qualis captarum esse urbium solet....When they entered the gates, there was not, indeed, that uproar and terror as is typical of captured cites.
What is esse doing in this sentence? Solet seems to be able to do the job by itself.
You can tell by looking verbs up in a good dictionary that provides copious examples, such as Lewis & Short, which is available on-line, or the Oxford Latin Dictionary; or, eventually, by substantial reading. But in this instance, even in English translation something is missing without a complement: "there is accustomed . . . " needs "to be".