You can tell that 31 out of 33 is in its simplest form by checking if the numbers have a common factor that is not 1, if the numbers do have a common factor that is not 1 then the fraction is not in its simplest form but if the numbers do not have a common factor that is not 1 then the fraction is in its simplest form.So, in this case you would check if the numbers 31 and 33 have a common factor that is not 1, if the numbers do have a common factor that is not 1 then the fraction is not in its simplest form but if the numbers 31 and 33 do not have a common factor that is not 1 then the fraction is in its simplest form and you can see by doing that you can tell that 31 out of 33 is in its simplest form because 31 and 33 do not have a common factor that is not 1.