OK, there are many ways to go at this. Depending on the convenience vs. energy usage, your control scheme will vary. Just remember that the tiny amount of pump power controls a large amount of tankless heater usage. The heater is your major energy source, and will lag the pump by a certain time constant. A certain amount of time will pass before hot water starts coming out of the heater, and this adds to the time it takes for the water to travel through the plumbing. Likewise, at the end of the preheat cycle, the heater will run for some time after the pump shuts down. Considering an average home uses hot water a dozen times a day, a few extra seconds of heater runtime per cycle could add a substantial amount of energy usage, especially during "false alarm" cycles where no hot water is actually drawn from a tap.
That being said, most sophisticated home systems are rigged up like this:
This picture depicts a wireless setup, but your "remote sensors" can be wired if you like. The pump controller acts exactly like a motion sensing security light controller. If you are trying to save money, that's what I would use. Just make sure to find one with an adjustable lamp timer circuit and motion sensors that will not trip when a cat walks in.
Whatever control scheme you use, install an aquastat at the tap that feeds the return line. This one device will save the most energy, as it will automatically compensate for all kinds of unknown factors. Whatever your incoming water temperature, whether there is a false trigger or not, the aquastat cuts the return flow when there is hot water at the tap. It is relatively expensive (even more so if you hack/DIY), but will pay for itself in heat not wasted, over and over every cycle.