How to determine if my house is continuing to settle?
If you really want to go there, you need something that will serve as a reliable reference, which I don't see a "now angled trim piece" as fulfilling. It's what's known as a benchmark, and should be something that does NOT move (so if you think the building is moving, you need a benchmark that's not connected to the building.) If you stick with things in the house you can see any relative movement, but only that. A chimney foundation is a typical "relative in-house point."

Realize also that porches are never (properly) built level or flat - they may indeed settle due to substandard foundation, but when properly built in the first place they have a drainage slope away from the house.

Typical things that are used for benchmarks are a pin in exposed bedrock, a bridge abutment or similar solid foundation, or a well casing. Something that is not annually moved by frost... Official benchmarks can be found on topographic maps (marked BM plus an elevation) and you will usually find a 3-4" round marker with a punchmark for the actual point in real life. But unless one happens to be near your house, you would probably just try to pick something that is near your house that would serve as a local benchmark to see if your house is moving.

I would in general agree that if it's got apparent motion at 100 years old, something above the foundation is more likely, unless the soil is in some way unstable under it.