Don’t let your dog approach. Even if both are friendly, it is an unequal meeting and not fair to the on-lead dog. There is usually a reason the dog is on a lead: respect this, give space and move on.
2. Both dogs on lead
Ask permission first and wait for the answer. Even if the owner says it is OK, be careful: being on leads adds extra tension to any meeting. If the answer is no, respect this, give space and move on quickly and quietly.
3. Both dogs off lead
a. If close enough
Ask permission for your dog to meet the other and then apply same rules as 2.
b. If not close enough
Are there any obvious resources (toys, balls) that could cause friction? Yes? Then avoid potential conflict and move on.
Is the owner training the dog or playing with the dog? Yes? Then leave them in peace to enjoy their time together.
Is the dog reasonably matched to yours in age, mobility, size? Yes? Then it might make a suitable play mate – move closer and apply 3a.
All of this assumes good off-lead control of your dog. If you couldn’t follow this etiquette because your dog would have already been over to the other dog, then use a lead until you can. And why not use the dog training etiquette to drive your training? Here are some things you could to start work on:
Walk close – stay by your side on or off lead whatever the distractions.
Chase recall – call back your dog even if he has already decided he wants to say hi to the other (also useful when you come across livestock or wildlife).
Emergency down – stop your dog wherever he is when you see another dog coming.
Directional send away – great for teaching “full on” dogs to approach another more politely.
Play bow – teach your dog to invite rather than intrude.