One of the most frequently extolled virtues of welted shoe construction is that the sole can be relatively easily removed and replaced. By caring properly for the leather uppers, resting them between wears and resoling them when then required, your shoes really can last decades.
But according to the last user survey on reddit's r/goodyearwelt, the average user had 10 pairs of quality footwear and had never had a resole carried out - it appears to be an option that people are aware of, but they just don't take advantage of. I think there could be a few factors contributing to that:
1) Enthusiasts just own a lot of shoes, and with a large rotation it takes longer to wear them out
2) Even though it's a relatively small cost compared to the cost of buying new shoes, it can be off-putting have to put up cash for a resoling. If you're anything like me, you'll probably just leave them on the shelf and procrastinate getting them resoled
3) Not having easy access to a reliable cobbler - it can be a bit anxiety-inducing handing a really beloved pair over for work, and a lot of effort to post them
But if you can overcome the above, let's go through the tell-tale signs that a resoling is required.
Note that these are aimed at leather-soled dress shoes.
Holes
Starting off with the most severe symptom - this usually manifests as a hole at the point of the sole with the most wear and contact with the floor (usually around the ball of the foot). Typically there will be a ragged edge around the hole with some cork filling showing.
Using your shoes in this condition is a bit of an invitation to further, more severe damage, especially if you wear them in wet weather. This can lead to water penetrating the cork footbed and leeching into the surrounding materials. Since it's structural damage this will require a full resole, but if you really love the shoes it's a good investment.
Thin / compressed feeling
A preview stage to the above - you can feel the sole is becoming compressed and "thin", again at the point where the sole sees the most weight and contact with the ground.
You'll learn this through experience, but you can feel the sole "give" under pressure if you poke it with a finger, whereas the surrounding area will feel firmer. You may also be to distinguish a hollower sound if you tap it with a fingernail compared to a less worn area. It can be hard to identify visually, but if inspected carefully you may be able to see that the worn part of the sole looks "flatter" than the surrounding area.
So what's happening here? Well in addition to the leather of the sole wearing down, the cork filling in the footbed is being compressed and disintegrating through normal wear - as such you may also feel less support and comfort when walking.
This is also why can't just keep replacing a rubber topy sole over your leather sole in lieu of full resoles - even though the leather is protected by the rubber, the cork of the footbed is still being squashed over time.
Welt stitching visible / wearing
Unless your shoes have closed channel soles, you'll be able to see the welt stitch running around the perimeter of the sole. As you walk, the sole will generally wear away most at the ball of the foot and the very front edge of the sole, where you place the weight of your foot and then push off when walking. As such you can usually gauge when a resole is needed as the welt stitching has worn away visibly at the tip of the shoe.
Compare the stitching at point A around the side with the wear at point B to get a feel for this:
Rubber heel tip worn
A bit of a "canary in the mine" - as well as the sole section wearing down, you can also observe the rubber heel counter being worn away over time. This is pretty obvious at a glance, and you may notice the shoes can "rock" on the heels without a flat base.
This doesn't necessarily mean a whole resale is needed - your cobbler may suggest just replacing the heel tip, which is a cheaper option. It really depends on your own gait, what surfaces you've been walking on, etc.