Opens from new places or programs can hint at another reader, but the same person often reads on a phone, a laptop and an office computer, so such a pattern is no proof. Some mail programs also drop or rewrite images and links when forwarding.
If it matters who reads a message, send it to each person separately: from the HawkOpen dashboard every recipient gets their own copy, and a tracked copy has its own tracker. A copy is tracked only where your company has switched tracking on and the recipient has consented.