Here we are primarily concerned with monitoring technical standards, as opposed to time and budget which may be tracked by control charts.
Inspection is the most obvious technique - simply observing what is taking place.
Testing, such as pressure, stress and performance testing are other techniques. Establish a schedule of interim progress reviews. It can be on a fixed timetable or keyed to the completion of work packages. Obviously if you have appointed project team leaders you can perform reviews with them on a regular basis. This will keep them up to date and prevent complaints that you have gone off track.