How can I gain the trust of a dog?
Sure, to establish trust with an unfamiliar dog, first:

1. Make sure the owner is OK with you approaching their dog. Some dogs are just not sociable, and you should respect the owner's wishes as well as the dog's space, otherwise you will be in potential danger. 
2. Approach the dog calmly and at an angle. Do not make direct eye contact as it could be seen as a sign of aggression.
3. Hold out your hand to where the dog can see it and let the dog smell it first, so that it knows that you mean no harm.
4. If the dog does not display any signs of aggression, you can then try to pet it as it has established a basic level of trust with you.
What are the signs that a particular dog is not trustworthy?
There are a few signs that a dog may not be trustworthy, including:

   1. Growling or snarling: This is a clear indication that the dog is feeling threatened or aggressive.

   2. Barking excessively: If a dog barks at you excessively, it may be trying to warn you to stay away or it may be a sign of aggression.

   3. Raised hackles: When a dog's hackles (the fur along its back) are raised, it's usually a sign that the dog is feeling threatened or scared.

   4. Stiff body language: If a dog's body is stiff and its tail is straight or tucked under its body, it may be a sign that the dog is feeling uncomfortable or scared.

   5. Lunging or snapping: If a dog is lunging or snapping at you, it's definitely not trustworthy.

If you notice any of these signs, it's important to respect the dog's space and avoid approaching it. It's also important to note that even friendly dogs can display these behaviors if they feel threatened or scared, so it's always a good idea to ask the owner's permission before approaching any dog, and to approach the dog slowly and calmly to avoid triggering any negative behaviors.