Detecting OBS on steal attempts

[Paul L]

I'm the only ump and behind the plate. Runner at first attempts a steal of second. Catcher throws to fielder waiting on the first base side of second. Ball arrives ahead of runner, is caught, and fielder tags runner. It's an out, unless it's obstruction.

What should I be watching for? How can I tell from just in front of the plate whether the fielder is blocking the basepath. How can I tell if the runner was impeded when there is no contact before the ball arrives?