No problem.
Once you researched Circuit network, you will have 2 new items available to craft in the logistics tab, along the other combinators (Algo, decider, constant). Simply craft the one you need, and put it down inside the logistics coverage area of any roboport.
The blueish one (Unfulfilled Requests Detector) will output signals of everything the logistics network is currently being requested (by Buffer and Provider chests, as well as Spidertrons) and not currently being handled by logistic bots.
The green one (Player Requests Detector) will output signals of any player logistic request within the logistic coverage area of the network it is placed in that is not currently being handled by logistic bots.
They do need to be placed within the logistic coverage area of a roboport. If they are not part of a logistic network, they simply won't do anything. To check you can enable to Logistic Coverage button (The roboport Icon) on the map view and make sure the detector is within the orange area.
