Of course I must start this post off by saying always contact your vet if you have any concerns about your dog's health and well being. But I thought a general "first aid" and triage post might be useful for those events that always happen when your vet's office is closed.
The best place to start with first aid is to get to know your pet extremely well when they are healthy. Spend time getting to know your pet, and teaching him or her to allow you to hold them in positions that might be needed one day for first aid, ie their back, one side or the other, their paws, in between the toes, in the mouth and so on.
Know how their temperature feels. Yes this might sound odd, but know by heart the way their body feels when they're calm, excited and so on. This will later allow your senses to pick up on any heat an injury can send out. Also take your thumb and wrap their ear around it. If you place your thumb on (not in the ear canal) the inner portion of their ear, then wrap the ear around your thumb you can get a general idea of their temperature. Also know their paw pads can sweat, also a good sign of stress.
Another place to check is your dog's mouth. Take note of their gum color when they are healthy. When a dog is in stress (sick) their gums will/can show you if they're loosing any oxygen, and will be pale if not white or blue when you gently apply preassure and release.
These are just a few basics of course if you notice severe heat, a temp or blue gums your pet should see a trusted vet for full diagnostics. I'll share more tips later, but please feel free to share your tips too!