Signs of stress are not always obvious. Making sure you observe your dog to realise these subtle stress signals will help you recognise when your dog needs your help.
1 - Yawning
2 - Urination or Defecation
3 - Looking away or turning head to avoid eye contact
4 - Salivating
5 - Panting
6 - Nose licking
7 - Paw Lifting
8 - Turning or circling repetitively
9 - Repetitive Self grooming
10 - Pacing
11 - Showing teeth
12 - Growling
If your dog is stressed then remove them from the situation they are in (perhaps at the park and they don't like another dog) or if displaying signs at home then go to a vet for advise on what could be causing behavioural changes.