There's several ways. In our more casual FS that just went to the 5x/wk, one of the mages tracks it by marking daily visits they get on a spreadsheet by member. We're not really looking to quibble over less than the minimum, so if it only occurs sporadically for any given member, it's overlooked anyway. If that Mage notices a pattern (there's me +3Mages), the other 3 of us will pay attention for the next few days to a week. Since we're usually only focused on 1, maybe 2 players at the same time, we also note if we've seen them in game, chat, trader, etc. We may just send a note asking if they've forgotten us for visits; if their activity level has dropped over all, they'll get a note asking if they expect whatever rl issues must be keeping them away from the game to resolve soon; sent with a 'thinking of and missing you' tone. We are mainly hoping to avoid a gradual drop off of interest leading to someone eventually no longer playing.
My other FS (requires daily visits that were also tied to extra kp pts in our old Wonder spreadsheet swap; that's no longer the case since we went to NetZero) has a unique approach. There have always been 'visit trackers' and they always remain anonymous. If there is a need for more help with tracking, the AM will put out an FS wide message asking for more volunteers, but those volunteers are asked to contact the AM privately to keep the trackers anonymous.