I will agree with the others here and what they have said, but to answer your question.
Make sure that you already have her a whelping box in a quiet location. Take her temp daily...several times daily...a drop and constant in her temp will be a good indication that labor will begin soon. Also, she may either want to stay right by your side, or distant...depending on the bitch, I have seen both. Look for her to begin 'nesting' and wanting to stay near her spot or she may be seeking another 'spot' like a closet or under your bed.
Also, a sudden loss of appetite is a indicator as well (but doesn't ALWAYS) occur. And by all mean, do have an emergency number for an after hours vet handy...call and alert them now that you may be needing their services and what their procedures are should the case arise. Do as much research as you can between now and then.
Good luck and keep us posted!