There is a two fold problem. The first issue is, as you note, simply finding and mapping in detail the faults. That there was probably a significant fault somewhere along the southern side of the eastern Santa Monica Mountains was recognized by geologists as early as 1908. Mapping the fault in sufficient detail to be a useful guide about where or where not to build is even tougher. In areas with a cover of soil and vegetation it can be extemely difficult to find faults at all, let along map them in detail. Areas that have been built up are even tougher, since many subtle topographic hints of faulting will have been bulldozed and paved over.

The second problem is determining a likelihood that the fault is still active. Since the repeat time of quakes can be many decades or even many centuries, the fact that no historical quake has been known to occur does not mean the fault is inactive. The methods of Paleoseismology have only been developed since the '70s, and can only work in certain environoments. And the results are often approximate at best. In onshore areas this typically requires trenching, and even then may not be definitive. And perusading someone to dig a deep trench in an area of high real estate values can be even more challenging!