SWIM's not saying that there isn't a visual indication, just that he's never used it. SWIM's honestly never had an instance where he didn't get product from a pull, and he's pretty good at not using too much acid on limonene washes from fairly depleted bark; he doesn't like to waste time evaping water containing no product, so he makes sure to get it all in the few pulls he performs. SWIM also doesn't worry much about emulsions with nontoxic approaches, but somehow he never has to deal with much in the first place. SWIM doesn't specify visual indicators in his tek, because he doesn't use them.
SWIM really can't say much about the results of SWIY's naphtha pull, due to the lack of information. When SWIM used to use naphtha, he would use warmed naphtha and evaporate over a steam bath to reduce the volume, prior to freeze-preciping; precipitation would occur within hours.
SWIM is honestly having a hard time deciphering what the problem is, here. The best advice he can give is that what SWIY has is his material, his process, and his product. SWIM doesn't understand the doubt regarding the authenticity of SWIY's product, as SWIM's process is such that he can account for the product every step of the way and whip it into whatever form he likes to administer it in whatever way he pleases.
SWIM would like to help more, but the details of SWIY's process are scant and hard to follow. The very best indicator of the yield is to know exactly how much fumaric acid was used in FASW, then compare the weight of the final product to that amount; the weight will be the yield. SWIM doesn't know why alcohol was used in the extraction, and it almost sound like SWIY thinks that the emulsions (frothy layer between FASW and limonene) are the product? The fumarate product is dissolved in the entire water layer, and the emulsion are nothing more than bubbles.
Try this: Take the limonene pulls, shake vigorously with FASW until SWIY can't shake anymore, pop the whole thing in the freezer, once the bottom layer freezes solid, pour off the limonene, thaw the FASW, pour off leaving the excess fumaric acid behind, evaporate the water. If this doesn't yield a product, the problem is one of these things: Inadequate stirring of material in limonene, inadequate consistency/basicity of the source material, alcohol is holding onto the freebase.