I'm on to myself

Who's afraid of Virginia Woolf?のMARTHA:とGEORGE夫婦の関係はかなり悪い様です。
MARTHA: My arm has gotten tired whipping you.
GEORGE: [stares at her in disbelief]: You're mad.
MARTHA: For twenty-three years!
GEORGE: You're deluded ... Martha, you're deluded.
MARTHA: IT'S NOT WHAT I'VE WANTED.
GEORGE: I thought at least you were ... on to yourself. I didn't know. I ... didn't know.
MARTHA [anger taking over]: I'm on to myself
"be on to onself" は慣用句の様ですが、分からないので調べます。
"be on to onself" と言う慣用句は見つかりませんでしたが、"be on to" と言う慣用句がありました。
・Dictionary.com: Be aware of or have information about, as in They can't pull that trick again; we're on to them now.
・Wiktionary: (idiomatic) To figure out; to realize the truth.: I was planning it as a surprise, but I think he is on to me.
・American Heritage Dictionary: To be aware of or have information about: You'll never deceive us again; we're on to you.
これで上の引用個所の意味は一応通じます。　多分正解でしょう。