The issue with old building foundations is usually settling, which shows up as wall cracks. The underlying culprit is generally inadequate footings, if there are any. A solid foundation still cracks if it settles. Also, a solid foundation isn’t necessarily congruent with a seismic foundation. An earthquake can break any solid foundation.
Foundations designed for seismic activity include flexibility, and/or the capacity to allow for some building sliding back and forth, to ride out the shock waves of an earthquake.
Compression testing (as Sam Lee mentioned) came to mind when you asked about testing see how “solid” the old foundation is. Core samples of your foundation walls are crushed under a hydraulic press. Then you know the compression strength of your concrete (if that’s what the foundation was constructed with), but I don’t think that information is particularly useful. The foundation has been holding up the house for 120 years. It is probably solid enough.
What you want to know, for seismic activity, is how well the foundation sways sideways and rides up and down – to accommodate an earthquakes P and S waves. The solidness of a foundation doesn’t have much to do with its ability to handle a seismic activity.