I'm also having him microchipped. I was torn about that, because I've heard various pros and cons and I couldn't decide if it was worth it. But when they reminded me that he would be under when they did it, I thought, what the heck, let's just do it.
Forgive me if there's already a thread about this, but do you guys have your dogs chipped?
Unless you have a really odd brand scanner or chip, it should not be a problem. The two biggest chip companies are AKC and Avid and they can read each other's chips.
If someone really wants to keep your dog, they're going to remove that collar and throw it away. They can't throw away your chip. It's there to stay. You can prove ownership via the chip. Once the collar/tag is gone, it's gone.
I would imagine if you had adoption papers from a shelter/rescue group, old photos with you and the animal (things of that nature) would go a long ways towards proving ownership - especially if the person that had the animal does NOT have those things.
My dogs do not wear tags, but do all wear a collar with my mobile number embroidered on the collar and are microchipped.