if it's "you could tell Mary was evading our question" it means that you could see that she was trying her hardest not to answer the question by talking about something else.

The primary sentence is"You could tell Mary was evading our qusetion when she gave a circuitous response".I can not understand the structure "tell sb was".if it adds "that",I can understand it.Thank you for your help.

By the way, 'that' is omitted between 'tell' and 'Mary'. Doing so is entirely correct, but it might help your understanding if you imagine it there.

Thank you.I understand it after reading it for many times

It seems to be missing a few words in front like "We could..." or "You could...". Without any context, it has no meaning.