How do I establish, when looking at a tool, what grit to start the sharpening process on?
Really you don't need to think about it.
In practice you generally don't need to assess your edge, you can just sharpen the tool "as normal" because it will have blunted approximately equally each time1. This is pretty much the norm for working woodworkers and has been standard workshop practice for maybe the last couple of centuries, so no need to buck the trend :-)
One thing that can help a lot with this is not waiting for your tools to get blunt in the first place.
Little and often
This approach doesn't get stressed enough in sharpening guides, especially today and in online guides. It's common among carvers and some whittlers to keep their tools razor-sharp (literally) by regular 'top-up sharpening', generally by stropping.
Stropping is great for top-up sharpening because it's very fast and effective IF you haven't left it too long, 20-30 seconds to get back to shaving-sharp from a little blunt. So there is a lot to be said for adopting the same policy with plane irons and bench chisels also.
You can of course do the job on stones/plates if you prefer not to strop, but it's a little more difficult to do it as quickly! Still aim to be back at work in under three minutes. Worth bearing in mind also is that it's not always necessary to use your finest stones just because they're there2.
You might find some of these previous Answers useful to read over, in part to keep sharpening distinct from honing in your head (both sharpening tasks but usefully separated sometimes):
Sharpening grits -- naming and selection
What criteria would want me to bevel my chisel in a certain way
How does one aggressively sharpen chisels and plane irons when damaged?
1 This obviously assumes there hasn't been any edge damage, which might happen from a tool being dropped or from contact with a particularly hard knot, where minor folding or chips can occur. In this case you would need to sharpen more aggressively, but this is of course rare in day-to-day woodworking for most of us.
2 We should remember that it was common in the past for there to be a couple of oilstones (sometimes only one!) and maybe a strop to be the sum total of each woodworker's sharpening equipment. The problem these days is that so many of us leisure woodworkers are over-supplied with stones and plates to pick from, especially at the stupidly high end, when all most of us actually need for general honing duties is one combination stone (or individual medium and fine stones) and either a strop or a very fine stone when you need an edge to be more refined, very useful on chisels but with plane irons very much not always necessary.
On a plane iron you can work with an edge straight from 250 grit paper as Paul Sellers has commented on in one or two of his videos, which closely equates with some of the edges achieved in the past..... and we have only to look at old furniture made in workshops from these eras to see what those edges could do, an excellent practical illustration of what is good enough.