I always check the config.txt file of an asset (route, session, object) to see if the original creator has placed any restrictions or permissions there - usually in a license tag or in the description tag.
If it says may be modified for personal use only then go ahead.
If it says may be modified and redistributed as long as recognition is given to the original creator then you have permission to modify and redistribute (if you wanted to).
It is says nothing then you may modify but not redistribute.
There has been the very rare asset where the original creator has explicitly stated that modifications are not allowed but I do not see how that could be enforced for personal use only.
My (not legally binding) opinions.