This is what happens when someone tries to join:
[11:29:32] [user Authenticator #1/INFO]: UUID of player WhovianMan is 1bd9700a-dea8-4e67-9a0e-ae73170f87b9
[11:30:09] [server thread/INFO]: WhovianMan lost connection: TextComponent{text='Disconnected', siblings=[], style=Style{hasParent=false, color=null, bold=null, italic=null, underlined=null, obfuscated=null, clickEvent=null, hoverEvent=null}}
[11:30:09] [server thread/INFO]: WhovianMan left the game
And they do not get a message about you needing FML installed. I remember in 1.6.4 there was a message.