In the past, put small diluted amounts on an animal's skin, then their tongue, then have the animal eat it, then when you're satisfied you do the same with a person.
The animal method would definitely not be flawless though. Trying to eat everything a ruminant eats is a path to not feeling great, or if you're checking if something can be safely eaten by your dog, you might miss out on some great food.
I think tastes can be adapted, and culture will defined what is "great" food. But to extend your point, I wonder what happens if they encounter something completely non-toxic to dog, but are toxic to human.
I think that's reversing cause and effect. The only reason wolves stuck around long enough for us to turn them into dogs is because we shared food with them. So we were doing it before they would have let us experiment on them.
Pre-history humans weren't scientists who would've separated this test out from the rest of their lives. You're thinking too literally.
Pre-history humans would live close to their dogs. There likely wasn't a conscious "I'll give this to Fido, and if they're good in a bit, I'll eat the rest". Instead, a dog grabbed some scraps here or there, before or after the meal. If still hungry, the dog might wander around looking for more scraps and munch on what they find.
Eventually, the food you know you can eat runs out during a hard winter or other crisis, but you see Fido munching on some things like he always does. Maybe you'll munch on those things too. Congratulations, you've acquired enough calories to make it to the next genetic lottery!
Assuming the crisis is bad enough for you to experiment but not so bad for you to eat Fido, Fido survives until the next genetic lottery too! And so we have yet another co-evolutionary moment folded into our genomes.
If anything, if you had a dog 100+ years ago, it's because it provided value to you, in the arena of herding, hunting, work, and protection. Survival value, not anything close to modern notions of pets. That means your dog was a valuable asset, keeping you and your family from starving.
Royalty and rich people got food tasters, but only a dumb rat bastard would treat a dog like that. Plus, dogs have shorter intestines and more potent stomach acid, rendering safe many food sources humans cannot eat without getting sick. After seeing a dog chow down on a rotten carcass, you're not going to think of them for testing the edibility or safety of things.
That's not to say dogs haven't died like that through history, but it's not a thing people would do deliberately. Food used to be hugely more scarce than it is now, and a large majority of humanity got by with very little. To use a dog for testing new food would have been foolish or cruel.
Dog breeds specifically tailored for pets have been around for centuries. Go to an art gallery… you’ll find countless portraits of people from centuries past with their ‘lap dogs’, pet squirrels, pet cats, etc.
