A back titration solving a soap sample in a volume of mineral acid with a known concentration, and then performing a titration on the remaining acid with strong alkali can give an approximate measurement of total fatty materials.
Beware the fact that soap may contain inorganic salts that can act as pH buffers, causing interference in acid-base titration. Presence of certain surfactants such as ionic/zwitterionic ones may lead to increased error in measurement. One possible approach is to emulate a blank sample with the same inorganic salt/surfactant concentration as the soap (if formula of soap is completely known), then subtracting the blank to the sample measure.