In the olden days we'd have the charts out and were much more aware of FIR boundaries and changeover points.
Jeppenson EFB enroute product is very good for all that. Boundary's marked if you want and you can one finger a boundary waypoint and you can see the relevant frequency's from both sides.
Apart from it sucking battery like a swine it works very well, allows you to ask for sensible directs to the edges of FIR's which in my experience increases the likely hood of getting the instant reply of "approved"