Just thought I'd mention, I have a terrier breed and they don't seem to do the scratching of the bed, panting etc that you would expect - at least mine haven't ;) Bedlies in general are very stoical.
They do pant afterwards tho, when the pups are suckling and while the womb is contracting down again.
I look for loose motions and a clingy, uncomfortable bitch, then I know we're likely to 'go' that day, but the first obvious sign is when the bitch starts to have real contractions

Don't know if this is typical of all terriers tho ?
With Brainless on this, if in doubt get her checked by the vet ;)