The short answer...Pull the drums, clean them, along with the brake plates, removing all traces of dirt, brake dust, oil & grease. Measure the ID, if it's close to, or beyond 111mm...or/and you find major rust pitting, it's time to replace the drums. They probably won't be beyond 109-110mm. Source new shoes, front & rear, preferably with grooved friction material; that helps to keep brake dust from accumulating between the friction surfaces.
For tech discussion purposes, let's assume that you have them properly adjusted, are getting full lever force to the brake cams, and the plate assemblies are not binding in any way.
That leaves two issues: shoe contact and contamination. The former is easy enough to figure out; clean & scuff the friction material, make a couple of test stops, then see how much of the surface is developing a normal, glazed, appearance. It should be 100%. This is more of an issue with the combination of new shoes and worn drums. The greater the ID, the bigger the drum arc...and the farther the new shoes have to wear-down toward their centers to get full contact. That's why there's a max ID spec (111mm).
Contamination is sometimes not obvious, especially with 1970s-vintage, original, brake shoes...as OLD CT mentioned. That said, grease/oil can penetrate the friction linings and it doesn't take much to kill braking power. I've seen original brake shoes that still worked as well as they ever did...and others that looked like new but were totally worthless, even after cleaning & scuffing. FYI, Brakleen is only effective if used before petroleum has gotten more than surface-deep. Otherwise, it'll be a short-term/semi-effective fix. The residual oil will be drawn to the surface by capillary action. Normal braking heat will speed that process.
Rust is also a surface contaminant that reduces friction. Best way to remove rust, from drum surfaces, is low-pressure bead blasting. It's thorough and removes less solid metal than any other mechanical process. Unless it's substantial (and it rarely is) the impact of pitting on stopping power is minimal. Scoring, OTOH, doesn't have anywhere near the impact that intuition says it should have. As long as drum ID isn't near, or beyond, the limit, the main effect of scoring is increased break-in mileage for new shoes. I wouldn't want scored drums but, scoring alone doesn't necessarily mean that they're suitable only for paperweights, model boat anchors or recycling.
The final judge of success is actual braking performance. It's a foolproof way of compensating for lack of technical/mechanical expertise. Braking power, into the low 50mph range, will be pretty decent...if everything is working & adjusted as it should be. You should be able to lock the rear wheel, though I'd only ever try that as a test and at very low speed...it's brutal on the machinery. Up-front, forget trying to pull a stoppie. It's a lot safer, not to mention easier, to test front brake power at a dead stop. Grab a handful of front brake lever and attempt to push the bike forward. With optimal brake action, the front wheel should remain locked, or at least damned close to it. FYI, break-in, typically called "bedding-in" of brake shoes/pads is part of the adjustment process. Even new drums & shoes won't perform optimally until this has been done. So take this into account, if anything has been replaced, it can make a BIG difference.