You can identify possible aggression in your dog by recognizing any of the following signs in his/her body language. This is likely to apply to all dogs, but it's possible that not all of these signs will present at the same time:

Head is up and ears are forward - the dog is alert. Snarling/showing teeth. Hackles (hair down the dog's spine) raised. Tail is up.

Try to look for more than one of these signs before addressing appropriately to deal with aggression as alone, some of these may not indicate aggression at all (i.e. tail up or ears forward may not indicate aggression, but hackles up or showing teeth are a good indication of it).

At all times when addressing a dog's behavior, be sure you can accurately determine what the dog's state of mind is. Why is the dog doing what it's doing? This will help you to determine what action is best to take.

The best way to avoid being attacked by a dog that appears to be aggressive is to simply ignore it.
If you stop in terror or otherwise exhibit fear in any way, believe it or not, the dog will know it. Doing so makes you a weak target for the dog - i.e. prey.

A dog who is barking at people walking past the gate is more likely - should it get out - to bite a person who freezes in fear than someone who is just walking past and ignoring it.