Use 'surmise' to convey you're making a conclusion without solid proof, rather than a random guess. Seeing the empty driveway, I surmised they weren't home.
Pro Tip 2/3
Implies Thought
'Surmise' suggests you've considered the evidence you do have before reaching your conclusion. Judging by the loud noise, I surmise the machine is broken.
Pro Tip 3/3
Formal Context
The verb 'surmise' is found in more formal or literary contexts than everyday conversation. Investigators surmised that the painting was a forgery.