When is it appropriate to ask whether relocation costs would be covered?
Reimbursement for relocation expenses is part of your compensation for the job.  The best time to ask about it is actually when you are negotiating over your salary.  Depending on the policies of the hiring institution, it may or may not be possible to negotiate over the amount of the relocation expenses.  However, whether or not you can negotiate for a larger reimbursement, you would probably want to find out how much your employer is going to be paying for you to move before you agree to take the job; long-distance and international moves can be quite expensive.

If you have already agreed to take the job, but you have not previously asked about relocation expenses, you ought to do so immediately.  I would contact the head of the department and ask them.  They may give you an answer immediately, or they may refer you to human resources.  However, there is no reason to hold back on asking about this.