The fourth line down from the top of the display is the "Department/Site" line. When the scanner is monitoring the control channel (as in no active transmissions), note the department/site alpha tag and whether there are any bars showing in the signal strength meter. The scanner dwells on those sites it "hears" a control channel for about 1.5-2 seconds, so you'll have to look quick.
Signal strength indication means it is a site your scanner is receiving the control channel and you are capable of receiving traffic. If it was me, I'd keep monitoring it.
If you see the department/site tag "whizz by" (as in NOT spending that 1.5-2 seconds), the scanner isn't receiving the site's control channel. It would be ripe for avoiding or some other way (such as a quick key assignment to off).
I'm a little handicapped because I don't know the Starcom 21 system very well. That said, if your District 11 Favorites List has the same, or at least some of the same, sites as does your District 18 Favorites List, then you have an opportunity to improve efficiency.
If that was the case, I'd recommend consolidating these into one Favorites List and setting it up such that District 11's talkgroups are in one Department and District 18's talkgroups are in another Department.
By doing so, you'd avoid redundant site monitoring. Each site you "redundantly monitor" costs you 1.5-2 seconds of scan time.