I remember LUG's Starfleet Academy Box, they had a very detailed description of the Sol system. I think the perimeter is quite a solid one they even provided a template of the System and presented a sphere of dotted lines where the Perimeter is. On the other hand I think there will be certainly sensor arrays at least as Far out as Pluto to get a clear View.
I also think Starfleet Command will receive Data from other Systems and Starfleet vessels as well. So they get to be warned well before the enemy get to sector 001