I have no firsthand experience with this trail but a quick search of the Ventanawild.org site turned up a couple of posts from different folks either traveling out to the Potrero itself or a few folks trying to find/follow the Lottie Potrero Trail.
Sounds faint and unmaintained at a minimum... Others described it simply as... "lost."
There was a posting from May 2010 that suggested there was some confusion about whether the area around Baldwin Ranch was open to the public which apparently called into question the ability to gain access to the Lottie Potrero TH (there were both "trail" signs and "no trespassing" signs.
Anyway, just recently (earlier this month), it looks like someone else was able to successfully find and follow the Lottie Potrero Trail. They reported it as brushy and at times, hard to find, but that there were sections of obvious historic tread. Interestingly, they said the first 1/2 mile or so of trail from the Potrero heading down is not correctly shown on the maps (which probably is the cause for most of the confusion in trying to find/follow the trail. They give some insight into where to instead correctly find the trailhead.
See link below for some hits on this trail. The first "hit" is the most recent update with the successful hike of the Lottie Potrero Trail.
html code not working, so here's the direct web address for you to copy and paste: