It's a fun idea. I do keep a bit of an open mind towards the theory. But, I enjoy the theories more for entertainment.
If Aliens did help our advancement, this is a big world. And it could take a longtime to find anything concrete to prove it with.
Though, there are many hints in history. That our ancestors had knowledge they shouldn't have been able to possess. There are some interesting documentaries that cover this well.
I can't really think of anything that is striking evidence for Aliens influencing our past. There's no one thing. It's more like a collection of little things. Particularly, how many ancient structures seem beyond our ancestors ability to create them. Such as the pyramids, which you can find not only in Egypt, but in other civilizations as well.
There's also structures that suggest an understanding of mathematics beyond the knowledge of our ancestors. This, is only scratching the surface.
Some myths in a variety of cultures too, imply we were visited by otherworldly visitors from the stars. Whether or not they're is a grain of truth to them, well who knows.
If Aliens did influence our development, it's possible a large portion of our ancient deities (if not all) were Alien cultures. Zeus, could've been an Alien with a ray gun.
Well, it's a fun idea to entertain anyways. But, I'm a writer. So I deal in the currency of wild theories and ideas. ^^ So, I don't mind if it's farfetched. It entertains me.