No, this problem is faced by anyone that needs to read code code written by other developers.
If you are faced by a code base written by a team of 30+ developers, good luck trying to find out which types implement which interface.
At least LiteIDE (http://code.google.com/p/liteide) is a possible help that eases the problem.
I can't help to think like what @tomp said. It's really up to the developers to annotate their code and document their Interfaces, specially when using a systems programming language like GO, comments are essential as the code is not as self-explanatory like Python or Ruby. GO provides an excellent tool to generate documentation from internal or external packages: http://golang.org/cmd/godoc/.
There's this great write-up on GO's Interface by Jordan Orelli: http://jordanorelli.tumblr.com/post/32665860244/how-to-use-i...
No, it's not a problem for code readers. If you see a file being passed as an argument to a function which expects a Reader, you know that it implements Reader. Otherwise it wouldn't have compiled.
Second, you can search for the function definition(s) that define the interface. Search for Write(p []byte) if you're looking for Writers.
If you're in control of the interface, you can just modify it slightly and see what breaks.
The thing about interfaces is that the function that takes the interface is the thing that defines how it is used. The implementation of the type that implements the interface is irrelevant.
log.Logger writes logging output to an io.Writer. It doesn't care what the implementation of any particular writer is... the writer could be sending data across the network, it could be writing it to the console, could be writing to a file... doesn't matter. Logger just calls Write() and merrily goes about its business. Trying to look up all the implementations of the io.Writer interface is not useful when looking at log.Logger. They're mostly unrelated.
Now, if you're trying to figure out how to do some specific thing, like log to a Windows named pipe, then you'd want to start looking around your named pipe library to see if it implements Write().... which it almost certainly does.