The Pasuk says that Yisro was amazed at the fact that Hashem punished the Mitzrim for all the evil they did. The Pasuk uses a Lashon of Zadu which Rashi explains to mean evil done against Klal Yisroel. The Targum explains that the Lashon of Zadu, means everything they thought to do against Klal Yisroel. Meaning they were exacted with punishment for all their thoughts as well. The Brisker Rav comments and explains that a Goy is punished for an evil thought even without an action. He says that the actions that were done were known to all, however the plans and thoughts to do even more to klal Yisroel was only known to the advisors of Mitzrayim. Chazal tell us that Yisro was one of the advisors who was in charge of all that was done to klal Yisroel. Therefore it was specifically Yisro who was able to see the Yad Hashem, in a way that no one else could. He had the ability to see the Mida Keneged Mida even for the thoughts as well. This is the stress of the passuk that says that Yisro was able to recognize Hashem because of all the punishment he brought on Mitzrayim.