What it means
ところを見ると means “judging from” or “by observing.” It introduces an observable situation that provides evidence for the speaker’s inference. The expression commonly follows a plain-form verb clause and is used when the conclusion is based on what someone can see or otherwise recognize from the circumstances.
Usage note
ところを見ると emphasizes evidence from an observable situation, so it is not a general-purpose replacement for expressions meaning simply “because.” The polite counterpart 見ますと is used in formal or polite speech.
How to use it
窓が開いているところを見ますと、誰かが中にいます。
Judging from the open window, someone is inside.
窓が開いているところを見ると、誰かが中にいる。
Judging from the open window, someone is inside.
