〜そうだ — appearance or hearsay?

Your own impression from looking. Attaches to the *stem*, not the plain form.

いい becomes よさそう and ない becomes なさそう — irregular, and regularly tested. Never used for things you can see plainly: you don't say 赤そうです about something obviously red.

Reporting what you were told, without judging it. Attaches to the *plain* form, which is what separates it from the looks-like そうだ.

Compare: 雨が降るそうです (I hear it'll rain) versus 雨が降りそうです (it looks like rain).