Are dogs playing or fighting?

I recently acquired two new dogs, one a 10 week old Labrador and a 6 month old Dachshund. I already have a year-and-a-half old Cavalier King Charles Spaniel. I know that dogs play with each other and will "bite" one another. When should I actually break them up? The lab is teething at the moment and likes to bite practically anything and I'm worried that she may hurt the two smaller dogs. My Spaniel is a fragile little thing and I don't want to leave them alone for an hour and come back to find her all over the floor.