You must be joking. - Você deve estar brincando.
We use modal verbs of probability when talking about situations that we think are possible or true.
The modals below are used to express probability:
|Modal of probability||Level of certainty|
|must, have to||100 or almost 100|
|might, could, may||>0 <80|
|can't, mustn't, couldn't||0 or almost 0|
|He must be tired||Ele deve estar cansado|
|That should be the postman||Deve ser o carteiro|
|He might change his mind||Ele talvez mude de ideia|
|She can't be at home||Ela não pode estar em casa|
|Examples of using probability modals in the present and the past.||BBC Learning|