As already mentioned there are off the shelf tools for detecting various items which may be buried in interior walls. Timber studding, metal pipes and current carrying wires can all be detected and differentiated reasonably easily by fairly cheap detectors in close proximity.
Handheld detectors are available in DIY or more professional specs and are generally used when you need to drill into a wall to ensure that you either have a secure fixing point (eg studding rather than plasterboard) or to avoid hitting pipes or electrical wires.
Assessing structural integrity and strength is a bit of a different problem. Apart from anything else you need some idea of what you are looking for.
For example in a timber structure rot, fungal attack or termites may be the main concern, in steel structures you may be looking for corrosion, fatigue cracking, degradation of protective coatings or creep; in concrete cracks and corrosion of steel reinforcement are potential concerns as well as environmental and chemical degradation.
An inspection of a structure may also look at whether is has been constructed as designed eg have corners been cut by using inferior materials or omitting fasteners and whether the dimensions specified in the design have been followed correctly. There are also potential situations where apparently minor deviations form the design can have serious consequences for example the Hyatt Regency walkway collapse
This example illustrates that a simple evaluation of material properties may not
guarantee that a structure is sound or safe.
In terms of structures as a whole measuring resonant frequency may be a useful metric of overall structural integrity (especially for tall buildings and bridges which sway measurably with wind loads) here lasers or ultrasound may be able to provide very precise measurements although this does of course depend on having a baseline measurement to compare against.
It may also be useful to measure changes in dimensions over time particularity where is is suspected that buildings may b e sinking into their foundations here again very precise positional measurements are necessary. A very crude version of this is to simply glue a glass microscope slide over a crack in a structure if it breaks over time that indicates that the crack is growing or shrinking.
In addition drones, robots or endoscopes may be used to make visual inspections of otherwise inaccessible parts of a structure.
