The lowest possible hanging fruit for any library author is ensuring your code compiles with as many warnings enabled as possible. Sadly, GCC does not make this process simple as there is no one warning flag to rule them all, specifically because GCC has many warning flags that are not useful for modern versions of C++ (in other words, they are, in a sense, mutually exclusive). The best place to start is with the following warnings:

-Wall -Wextra -pedantic -Werror

This turns on most of the important warnings while ensuring that any warnings that your examples or tests compile will generate an error. For some libraries, however, this will not be enough. At the time of writing, the following are the flags that Microsoft's Guideline Support Library uses:

-Wall -Wcast-align -Wconversion -Wctor-dtor-privacy -Werror -Wextra -Wpedantic -Wshadow -Wsign-conversion

One additional warning that the GSL uses is conversion warnings, which will tell you when you convert between different integer types. If you are using Clang, this process can be a lot easier as it provides -Weverything. If weeding through all of the warnings that GCC provides is too much work, one approach to solving this issue is to make sure that your library compiles with the Clang compiler with this warning turned on, which will ensure your code compiles with most of the warnings that GCC provides. This way, your users will not have trouble with your library when they have to ensure specific warnings are enabled in their code as you will have tested as many of them as possible.