

If you get the SciFi Robots, you'll find no license on the Asset Store page, and no readme and no license.txt in the package.

If you get the Standard Assets provided by unity, you'll have to dig, find, read and understand the actual license, where there is no mention that you must credit the author If you get the "Unity-Chan!" Model, you'll have to display somewhere the "license logo or the license sign separately" as specified by the “Unity-Chan” License Terms and Conditions I'm assuming that by "free asset", you mean an asset available on the unity Asset Store that is shown when filtering by "free" ( resulting in something like this).
