How do pilots estimate glideslope visually without PAPIs? If my (enthusiast-level) understanding is correct, on instrument approach there are specific glideslope paths a plane is expected to follow. On ILS this is self-evident, but (again, if I read approach plates correctly), VOR DME approaches also entail a certain altitude that must be crossed at given DME distances. I assume, crews estimate their rate of descent based on their ground speed etc.

Now, when visually, apart from PAPI lights, is there any other way of estimating the correct glideslope? With C172s and 55 knots, I expect it's easier to simply crawl to the runway and adjust accordingly. But how does it work when you have a B737 or A320 landing on a 6000ft/1800mt runway without PAPI lights? I expect a certain kind of precision is required, but does it come merely with experience, or is there some other way?
There's no instrument you need for a good approach angle in good weather, it's all done by eye. When you train as a pilot you develop a sight picture of what a good approach looks like, and you develop a sense of what a runway looks like when you get too low or too high. It comes from experience and practice combining with the human brain's excellent spatial processing capabilities.