Well, if you receive a P45 from new employee and it states wk1 or month 1 then you tick the box - providing P45 is a recent one for current tax year.
If you receive a P46 from employee it depends whether they have ticked box A, B or C and on the reverse of the P46 it tells you if box A then emergency code cumulative, box B emergency code wk 1 or month 1, box C - BR code.
If you have brought forward an existing employee from previous tax year you apply the current tax code for the year but uncheck any employees who were wk 1 or month 1 for the start of the new tax year.
If you receive a P9 from HMRC for an employee it will tell you what tax code to use and whether to apply wk1/month1.
Hope this helps you.