Well, it really depends on where the tattoos are and where we need to treat. Our triangulation tattoos are placed on specific parts of the body, usually. Places where the skin is fairly tight against the bones or not very prone to moving. For instance, if you feel from your armpit down the side of your ribs you'll find a place around the middle of your torso where there's very little muscle or fat, and the skin is very close to the ribs. If you go towards the midline from there, you'll find that this generally aligns with the xiphoid process, the very tip of your sternum. So we would tattoo once on either side and then again right on midline on the sternum. And that could be used for treatments basically anywhere in the torso because we would align to those tattoos and then shift the field to the actual treatment area.
But if someone is tattooed in those areas there are a few ways we could handle it. The first is the simplest: just tattoo over it. If a small black dot would be visible through the tattoo, then we're fine. Second, we tattoo somewhere they don't have tattoos; this can make things more tricky but its doable. The third way, would depend on their tattoo, but if they have a tattoo with specific things on it (Ie an image of something rather than an abstract pattern) Then we can use some detail of the tattoo as our marking point. Use what's there. But as Omeletta said, UV ink would also be very useful in those kinds of instances because then we could just put a glowing dot in and not worry about tattoos they have.
Although to be honest, most of my patients are of the age where they generally didn't get tattoos, so it doesn't come up too often.
I'm based in Las Vegas, Nevada. I'm going to go to a place called "Diversity Tattoo" later this week and have them try a few inks on me, to get an idea of what the best case (Ie fair skin) looks like.