Same way you identify which wrench to use on a hex bolt: You get a feel for it over time and you find the one that “fits” right.
You should be able to lightly rest the Philips driver in the head of the screw, and the tip should touch bottom. If it doesn’t touch the bottom, go a size smaller. If you can rotate the driver in the screw head, go a size bigger.
That’s a major flaw with Phillips drive though; it isn’t that obvious. A #1 and and #3 driver will pretty much turn a #2 screw, unless the screw misbehaves and needs a little more torque to tighten or loosen, then you’re that much more likely to cam out and damage the screw, the driver or both. But Philips screws do that anyway even with the correct driver size, so.
Same way you identify which wrench to use on a hex bolt: You get a feel for it over time and you find the one that “fits” right.
You should be able to lightly rest the Philips driver in the head of the screw, and the tip should touch bottom. If it doesn’t touch the bottom, go a size smaller. If you can rotate the driver in the screw head, go a size bigger.
When in doubt, it’s probably a #2.
You know that is so obvious, but it makes me realise how unintentional I’ve been with my screws. I will do better.
That’s a major flaw with Phillips drive though; it isn’t that obvious. A #1 and and #3 driver will pretty much turn a #2 screw, unless the screw misbehaves and needs a little more torque to tighten or loosen, then you’re that much more likely to cam out and damage the screw, the driver or both. But Philips screws do that anyway even with the correct driver size, so.