Huge Subject
I think everybody is afraid to chime in lol. It's a huge subject and few answers are definite. I am curious for what purpose(s) do you plan on loading lead? I load with a lot of lead - the primary purpose is for 38 Special target shooting. Nothing shoots straighter than lead - at least, for my application. My hottest round is a 158gn SWC for 357 Magnum that runs about 1100 fps. It will visually start leading the barrel in about 30 rounds or so. When I'm shooting them, I infuse the occasional cylinder full of jacketed ammo to clear the lead. They say this damages the barrel. Well, I've been doing it for over 30 years - I'll let you know as soon as I start seeing barrel damage. In the course of this post, you're going to get claims of people running their lead slugs to over 1500 fps with no leading whatsoever, and on and on. Me, I've never shot lead without it leading - ever - under any circumstances. Lead leads - period. And then they'll go on about barrel fit, slugging the barrel, etc. Okay. Whatever. My barrels must all be the same size because they all behave the same with lead. And all my lead bullets are .358 - as 99+% of them out there for sale are - so what's the point in checking all this stuff?? You're going to end up right were you started. I digress. I'll try to answer your questions:
1. what is the best process for detecting barrel leading? Visual inspection, running a patch feeling for the lead fouling, or combination of both?
Visual. I use one of those flexible bore lights. They work great.
2. how often do you check for leading? after each shot, third, fifth?
In the field, I rarely check - unless I'm running an experiment with something hot. So my inspections are usually when I get home after a shoot and when I go to clean my gun (which is after every shoot).
3. how often do you clean the bore? after completion of each load?
After every shoot. Usually after about 200 rounds.
4. is it likely you will ever get to excessive pressure signs without first having encountered leading?
I doubt it. From my point of view, all lead bullets lead the barrel to some degree. Lead is soft (duh) and smears into the bore - that's all there is to it. But I have no need or desire to load any lead round that hot. I'd just use jacketed.
5. confirm you keep going until you; get leading, experience excessive pressure signs, or reach max load?
Talking about during a load work up? I go until I reach my purpose goal. Because my goals are realistic, I rarely run up against any of those to any major degree.
Maybe this will help some more. . . He's some of my lead loadings, and their purpose:
38 Special 148gn DEWC for IDPA Power Factor - 710 fps required to make PF. I use a Missouri Bullet Co (MoBuCo) soft cast slug (BHN 12). This is a soft slug that is extremely accurate. My worked up loads run about 750 fps (there's four of them actually - using Bullseye, AA2, W231, and Nitro 100). After about 200 rounds, the first 3/4" of the breech of the barrel has considerable leading - as does the cylinder throats. Actually, I think the leading plateaus after about 100 rounds of so; after that, the build up ceases.
38 Special 158gn SWC for ICORE Power Factor - 760 fps required to make PF. I use MoBuCo soft cast slug (BHN 12). Again, an extremely accurate round. They run about 790 fps (two recipes actually - using W231 and Nitro 100). As with the above round, leading is similar after about 200 rounds.
38 Special +P 158gn SWC for recoil practice. Runs about 920 fps. I use a MoBuCo hard cast slug (BHN 18) for the application. A really fun shooter. Doesn't lead the barrel quite as much as the above two loads.
357 Magnum. 158gn SWC for magnum level practice. Runs 1100 fps. Uses the same MoBuCo hard cast as above. A great shooting round, but does lead the barrel after only a few dozen rounds.
45 ACP. 200gn SWC. For target shooting. Runs 778 fps and is extremely accurate. Hardly leads the barrel at all. Bullet used varies - as long as it's a .452" slug.
Those are all my regular production lead rounds, and their purpose. I've got a few other ideas dancing around in my head - maybe some day.
As for other calibers:
I'll shoot 44 Specials with lead - but never settled on a recipe I liked (consistency issues, not leading) and haven't worked on a recipe as of lately. I rarely shoot my one 44 (Mag) gun. My next attempt will be with a 240gn SWC and probably Bullseye powder.
9mm: I run into leading problems. Although I've had decent luck with 147gn slugs and I'm going to pursue this one further. I think .356" is too small. Need to find some .357's. But. . . I rarely shoot 9mm and I have a lot of factory ammo.
10mm: Won't even consider it. (And I don't own a .40 cal)
That's just a brief rundown of my loading experience and lead. In short, I love shooting lead. It's accurate. Very accurate. When I'm shooting in a match, I'm packing lead. But I also have a realistic expectation of its high velocity performance. Like I said: Lead leads. It just does. I live with it. Some days I'm not in the mood to scrub lead out from my barrel; so I reach for my plated rounds that day - it happens. (I have plated equivalent workups for all the above-mentioned recipes)