To the best of my knowledge the earliest source for the concept of maaseh avos siman l'banim is Ramban on Bereshis 12:6. I would like to know if there is an earlier source and what the parameters of this concept are. Are all actions of the Avos, good or bad, considered simanim for their children? If no, how can we determine which are included in this principle and which are not?
I think Tanchuma Parshas Lech Lecha 9 - which says א"ר יהושע דסכנין סימן נתן לו הקב"ה לאברהם שכל מה שאירע לו אירע לבניו is the earliest source.
Rabbi David Cohen has published 5 volumes of Maaseh Avos Siman L'banim where he goes through at length what is considered a Siman. It is way too long to answer over here. Here are links to his Sefarim.
Well Medrash Rabbah also discusses the idea: Gen Rabbah 40:06 on Gen 12:16 for example. The idea is somewhat suggested at Gen Rabbah 48:07 (on Gen 18:01) and Gen Rabbah 54:05 (on Gen 21:30).
As @Gershon Gold just observed [just 3.5 years ago, actually], Medrash Tanchuma Lech Lecha 9 (on Gen 14:01) suggests the idea.
And Rashi, who just beat the Ramban to the ribbon, suggests the idea at Gen 18:01.