be on to

Verb

idiomatic

(idiomatic) To figure out; to realize the truth.

Japanese Meaning

気づく / 真実を見抜く / 理解する

After testing different configurations, the engineer realized they might be on to the reason why the server crashed.

さまざまな設定を試した結果、エンジニアはサーバーがクラッシュした原因を突き止めかけていることに気づいた。

Sense（1）

(idiomatic) To figure out; to realize the truth.

I was planning it as a surprise, but I think he is on to me.

is on to

（ present singular third-person ）

being on to

（ participle present ）

was on to

（ past ）

been on to

（ participle past ）