how do i know if i am infringing other people's logo?

If there's already a logo out there for something, somewhere, and it has the copyright, trademark, or restricted symbol next to it, then you can't use it!
Copyright: (C)
Trademark: (TM)
Restricted: (R)
... I hope this helps! ^_^

If it's been created, you can see it, it's out there, you did a search for it, it exists, you use it, you are infringing. Period.