In order to know if you are well shod, several points should be checked.
The first point, surely the most obvious, is that you should not feel excessive pain. Indeed, it sometimes happens that clients put on shoes that are too small (although this is quite rare), telling themselves that this is normal at the beginning and that the shoe will fit over the course of ports. However, if you feel pain, particularly in the heel and/or toes, this is a sign that the size is too small. On the first fitting, it is normal to feel some pressure in these areas, but not pain. Of course, your toes should not touch the tip of the shoe, because a shoe will expand in the pressure areas, but it will never get longer.
Next, when you try on the shoe: get moving. Take a few steps and check that your heel doesn't pop out of your shoes when you walk. This would be a sign that your shoe size is too big. (n.b. You should not be able to slip your fingers between your heel and the back of the shoe, as you sometimes read).
Once you have checked these first two points, which are problems linked to the size alone, you need to make sure that the last is right for your foot. To do this, make sure that there is not too much empty space at the metatarsophalangeal joint: this would cause unsightly creases in the leather over time. On the other hand, make sure you are not compressed at this level. A slight tension should be felt at the level of the guarantors (foot strike) but you should not feel excessive pain. This would mean that the upper is too low/fine for your foot or that your instep is quite strong: if this is the case, try a higher/wider last or open lacing models, such as a Derby.
Finally, make sure there is not too much empty space in the arch of the foot: this would indicate a last that is too wide for your foot.