The "trial flight" if we are going to use that term, is simply one lesson from the syllabus. A lot of people have this lesson, then don't bother to continue for any more, fair enough, that's their choice. For those who do, if we as FEs did that flight as an FI, then we have carried out instruction for a PPL for that person, so, no we can't test them. As to progress checks, that's not necessarily instruction, not sure, will have to try and have a look at that. Perhaps someone else will be along in the meantime?
You could of course just get students not to bother putting their "trial flight" with you, into their logbook when they comeback for their second lesson.
