You mean the way the do it with fully loaded long-range freighters? Start a stopwatch at the beginning of the take-off roll and see if it takes 40 or 45 seconds to reach V1? Not really practicable with something like a Citation Excel. A lightly loaded one like this will reach V1 in less than 10 seconds from a static thrust takeoff, followed by Vr within the next second. No time to look at a stopwatch or anything but really essential instruments.