You can try read-char-no-hang, but that will likely block until Enter is pressed.
In addition to the obvious implementation-specific details, this also depends on which OS you are using and exactly where you are typing.
For example, a linux terminal must be in "raw mode" or else it buffers text into lines.
If you can't find any allegro-specific instructions, then Linedit might provide some inspiration.
http://common-lisp.net/project/linedit/