for the third principal part? If I were confronted with “peperi” it would never occur to me to even look anywhere near “pario”, and several verbs have non-guessable perfect forms, yet my dictionary, as I believe is common, only has entries for the first and fourth parts (which is odd since the supine can nearly always be inferred). I have no idea how irregular perfect forms were looked up before Google!
Just input “peperi” (or any other Latin word) into Whitaker to get the full picture -
http://lysy2.archives.nd.edu/cgi-bin/words.exe
You either made an educated guess, or consulted an alphabetical list of irregular verbs (showing their principal parts), or made sure you were reasonably well acquainted with that list to begin with - in ascending order of desirability.
There are some patterns if you think about it. With practice you recognize something like, say, “spopondisse” as a reduplicated verb even if you don’t know the verb and you’d know to look for something like “spondo”. Most perfects besides totally irregular ones like ferro/tuli/latus are formed by adding -v to the present stem, by dropping the stem-vowel for -u- (mereo/merui), reduplication, adding -s to the stem, or lengthening the root vowel. A handful are form perfects in an irregular way but still close to the stem (peto, petere, petivi).
FWIW, John Traupman’s New College Latin and English Dictionary (Bantam) prints many of the hard-to-guess perfect forms. For example, I see on p. 305:
pependi
peperci
and peperi
Most of the time Traupman tells me what I need. Traupman also has tables of conjugations and declensions in the front pages. Moreover, he translates the examples into English. This is very helpful when the idiomatic meanings are non-obvious.
Another poster mentioned Whitaker. Besides the online Whitaker cited, there is also a free downloadable Whitaker, that lets you look up the words when offline. I use Whitaker very often.
Since I can read French fairly well, I also use the student Gaffiott, which I find helpful for its examples. They seem exceptionally well chosen, and in the student version of Gaffiott the examples are translated (into French of course).
Download Whitaker here:
http://archives.nd.edu/whitaker/words.htm
The Barnes & Noble here in Greensboro, North Carolina, keeps Traupman in stock. Copies also turn up in used bookstores.
I forget where I got my Gaffiott, but ISTR Amazon.