A drop in temp is usually a good indication of this . Is she doing any nesting at all?
Thank you for that information. The fact that she bred with a larger dog may present problems for her so I would suggest you keep a vet and emergency vet number on the fridge for easy access in case you have to be talked through anything, or need to make an emergency trip.
The dog may go into labor between 12 and 24 hours from the temp drop. When she seems anxious , looking for a place to have the pups, keep a close eye on her and only take her out on a leash. Some people make the mistake of letting the dog out thinking it has to potty and it ends up dropping a pup outside.
When mom starts active labor pains she should deliver the first pup within two hours, any longer and there may be a problem. There after she will have a pup every half hour or up to four hours.
I am going to direct you to sites that talks of delivery and what constitutes an emergency.