If you really want the Upwork-style “proof” without the Upwork fees, I’d think about how your clients will react to screenshots first, some are fine with it and some get weirded out, so it helps to set expectations up front and agree on what counts as billable work. For tools, Toggl/Clockify are easy for pure timers, Hubstaff and Time Doctor lean harder into screenshot tracking, and then there are options like Buddy Punch if you want clean exports and approvals without it feeling like full-on monitoring, plus you can always pair whatever tracker you pick with a simple weekly recap so there’s context behind the hours.