THANK YOU! What a great rule of thumb!
Of course, it requires of me to be able to understand if the stem means anything in itself, but that can sometimes be checked with Google translate if I don’t understand it.
However, I guess one can be fooled as well, even with the example hanashimasu, since hana means “flower”, so then my stupid ass would go: “oh, it must mean flowergardening with an extra し in it!” or something. 
Also, I knew the one two-or-more-kanji-rule, but forgot about it.
Awesome, thanks!