You cannot tell from one look. A healing sore gets narrower, its edge softens from angry red to pale pink, and it hurts less. The only reliable test is to measure its width twice, two days apart, and compare.
Almost every answer to this question is a list of things to look for, which is useless in practice, because all of them are gradual. A sore does not stop looking like an angry hole one morning. It narrows by half a millimetre a day, which is invisible to someone checking it in a bathroom mirror and obvious to anyone who wrote a number down.
So this page is in two halves: the signs, which are real but weak, and the test, which actually works.
Four things change, roughly in this order.
The pain drops first. Usually from around day four. This is the least reliable signal of the four, because it moves well ahead of the sore itself — people regularly conclude a sore has healed on the day it stopped hurting and then find it still there a week later.
The red ring fades. The inflamed halo around the crater goes from angry red to a duller pink. This is a genuine sign and easier to see than size, because colour does not need a ruler.
The white centre shrinks. A canker sore heals from the edges inward, so the pale floor gets smaller while the overall spot is still visible. A sore that is mostly pink with a small white middle is further along than one that is mostly white.
The whole thing gets narrower. The one that actually settles it, and the one nobody can judge by eye from memory.
Yes, and this is the part most people get wrong. A canker sore normally widens and hurts more for the first two to three days. Then it holds roughly still for a few days before it starts closing.
That plateau is where this question gets asked. Nothing is visibly improving, the pain has dulled but not gone, and it has been the same for three days — which reads as stuck, and is usually just the middle of a perfectly ordinary course. Getting worse early is expected. Getting worse after day four is not.
Measure the width at its widest point, in millimetres, write it down with the date, and do it again two days later.
That is the whole method. It works because it replaces a judgement your memory is bad at — is this smaller than it was? — with a comparison between two numbers, which is a judgement nobody can get wrong.
Widest point, in millimetres, two days apart.
Narrowing — that is healing
Down 2mm. A sore that is measurably smaller two days later is closing, whatever it still feels like.
Keep the numbers insteadTwo details make the difference between a measurement and a guess:
Smaller, flatter, paler, and less painful to touch — in that order of reliability, reading right to left. If you want one thing to look for rather than four, look at how much of the spot is still white: a crater filling in from its edges is the clearest visual sign that the sore is closing, and unlike size it is a change you can actually see between two days.
The first three are reasons to have a dentist look. The last two are worth mentioning to a doctor, because recurring ulcers in a fixed spot and ulcers with symptoms elsewhere are the cases where the mouth is not the whole story.
A minor canker sore shrinks by something like half a millimetre a day. Over two days that is one millimetre — real, measurable progress, and completely invisible to someone comparing today's mouth against a mental image of Tuesday's.
This is not a failure of attention. Nobody can hold a millimetre in their head. It is the reason Canker Core exists as a log rather than a checklist: two numbers and a date, taken twice, answer the question that no amount of staring in a mirror can.
Canker Core is a log, not a diagnosis. Nothing here is medical advice. A sore that has not started shrinking after two weeks, keeps coming back in the same spot, or comes with a fever or a rash is worth showing to a dentist or doctor.
Mark the sore, log its width and pain, and compare tomorrow. About fifteen seconds.