That's the trick about a smartzone system like Palmetto 800. You won't know at any given time if that talkgroup will work on that site or not.
The talkgroups that are carried on a particular site are determined by whether a subscriber radio (in this case, a radio in a Kershaw EMS ambulance vehicle) has affiliated with that site. The radios are designed to affiliate to the closest site, or rather, the site that the radio determines has the best signal, therefore the best chance of receiving and rebroadcasting the transmission from that radio. So, the Kershaw EMS talkgroup will be heard only on the sites that have a Kershaw EMS radio affiliated to it at that time.
There are limits to that process - not all talkgroups can affiliate to all sites. Agencies pay a monthly service fee to Palmetto 800 based on the number of subscriber radios, the number of talkgroups they use, and the number of sites they can access with those talkgroups. Only truly state-wide talkgroups can be carried "state wide". It's unknown to me whether Kershaw EMS pays to be able to access the Chesterfield tower with their talkgroups. Most agencies with multiple talkgroups have one that has state-wide access, typically called "Roam". The others are limited to one or a few local sites.
So you'll just have to listen to the Chesterfield site and see if you hear Kershaw EMS at all, and if so, how often. This is where a control channel decoding application comes in handy - you can park your radio on the Chesterfield CC frequency for days and let the software (such as Trunk88) log activity on the site. This can give you an indication of if and how often Kershaw EMS uses that site.
Hope that helps.