AFAICT, go build tags are a compile-time only construct, so there is no way to query whether a certain package was built with a specific tag. However, the 'netgo' tag triggers conditional compilation of the cgo_stubs.go file, and that file name ends up in the executable. (we could have looked for specific symbols in the 'net.a' library instead, but that requires locating it first, which is more hassle than it is worth) issue #93