The first sign of madness is talking to yourself.

Related:

• The first sign of a nervous breakdown is when you start thinking your work is terribly important.
  Milo...
• Since when is talking a sign of
  thinking...
• You're getting old when you stop to think and forget
  to start again...
• If you're ever stuck in some thick undergrowth, in your underwear,
  don't start thinking of what other words have "under"...
• When your friends begin to flatter you on how young you look,
  it's a sure sign you're getting old. -- Mark...
• To give of yourself,
  you must first know yourself...
• The first sign of age ... is when you go out into the streets .
  and recognize for the first time how young the policemen...
• If a loafer is not a nuisance to you,
  it is a sign that you are somewhat of a loafer yourself...