There are actually more precise (but sadly slow and relatively expensive) ways to deal with unusual forms.
I always try to make my students learn (slowly, and with examples of actual texts) the usual lists of verbs that come at all the good grammars (Like Smyth’s List of Verbs, Zinsmeister’s Alphabetisches Verbalverzeichnis, Adolf Kaegis’ Repetitionstabellen zur kurzgefaßten griechischen Schulgrammatik, Zuntz’s Summa Grammatica and the like.) And then drill them till exhaustion with the last 10-12 lessons of Zuntz’s Lehrgang which focus on the most weird verbal forms that come about relatively often in texts (note that the last 20 or so lessons dedicate a very long space to verbal forms in general).
But there’s always that one weird verb form you find while reading that leaves you puzzled. I do not disapprove at all of Perseus or any parser whatsoever, but precisely to avoid the creation of dependency I break my sacred rule of no dictionaries (assuming of course that by now they have learned to dislike the use of the dictionary as much as I do) and have them look for the form in one (or many) of the following materials and memorize the passage with the verb in question).
Assuming you don’t have a students commentary like Cameron’s Thucydides Book I: A Students’ Grammatical Commentary or a even better, a special Index like Stork’s Index of Verb Forms in Thucydides for the text you are working on, there are many good verb-form dictionaries out there that one can rely on:
+The biggest and most comprehensive is John J. Bodoh’s Index of Greek verb forms.
+Nino Marinone’s All the Greek Verbs.
+Georg Traut’s Lexikon über die Formen der griechischen Verba. There’s a free (but old) version here.
+William Veitch’s Greek verbs, irregular and defective.