It should be the law with all dogs that they have to be trained professionally. Professionals would be able to weed out the aggressive/non trainable ones in an instant.
While I agree with the theory - the practicalities would be very difficult. Unfortunately, dog training is a totally unregulated industry - literally anyone can call themselves a trainer or behaviourist. This leads to some people who are incompetent, sometimes dangerously so, fooling members of the public. These charlatans can give out advice that will make the dog worse.
Reforming the sector so that trainers and behaviourists have to have a specific qualification (e.g. APDT for trainers, APBC / CCAB for behaviourists) would go a long way to curbing this.
But we're also in a situation where there's a significant shortage of suitably qualified trainers and behaviourists. There always has been, but it's far worse now that all the 'pandemic puppies' are in their adolescent phase, when problematic behaviours tend to emerge.
There's no such thing as an untrainable dog - they can all learn, and they all do, with correctly applied positive reinforcement techniques. The difficulty, as any dog trainer will tell you, is getting dog owners to understand what they're meant to be doing, do it correctly, and do it consistently.
The reasons why a dog becomes aggressive are many and varied - genetics, poor socialisation, a bad experience (e.g. being attacked by another dog) and so on - but not all of those will be evident at a young age.
I'd support a dog licencing scheme if it came with compulsory classes - perhaps 3 weekly sessions of theory before you get the dog, and 6 weeks of practical sessions once you've got the dog (one-to-one for rescue dogs with issues), with follow up support too. But staffing it with suitably qualified staff would be a serious concern.
@lochmaree
@UndertonesOfCake that is really good to know about the collie stare thank you! I dont know much about dog behaviour but the stare always makes me feel uneasy and I'd read that if a dog is still and staring it can be a sign of aggression. and with it being loose with no owner nearby I feel like I need to be extra cautious. I have turned back before to avoid passing it.
No problem 
Staring is one of those things that needs to be taken in a wider context. Collies are a bit different from others, due to the use that staring has in herding. Sometimes a dog will stare because they're really interested in something, or curious about it. On one occasion I was stood at a bus stop with DDog and a girl was eating KFC while talking on the phone. I hadn't given this any thought until the girl loudly proclaimed on the phone that "that dog's freaking me out man he's proper staring at me". Needless to say he was staring at the bucket of fried chicken, not her though he could murder a bargain bucket given half a chance 
This is a really good video that shows you the signs that a dog is feeling uncomfortable about something
This is a reasonable video about the signs a dog is happy (I struggled to find a really good video!)
I'll take a moment at this point to debunk the idea that a wagging tail always means a happy dog - more accurately it's a state of emotional arousal - and like a stare it needs to be taken in context. However, a dog who is wagging its tail so hard that the hindquarters are wagging too, and is showing relaxed body language, is a happy dog.
Obviously I don't know this dog, and haven't seen it in action, and erring on the side of caution is always safer. Have you spoken to others (preferably dog people) in the village to see what they think of it? Chances are it'll be "Oh, Nell the collie, I do worry she'll be hit by a car when she's out wandering, but she's a nice enough dog, even if she did nick my dog's ball once" or something to that effect.