@Aritra It is impossible to be certain 100% if a neighbor city is dead. But there are a few things you can look for which will tell you in most cases if a player is active.
I see what you mean. I'm still playing solo but anyone in my neighborhood keeping track of trader and NH will see I'm definitely active.
It's amusing to me sometimes, there's another solo player (non FS) in my neighborhood that I see all the time in the trader (and about three others, but the one I'm thinking of occurs most often). We've never talked but we buy each others stuff often. (And apologies if "he" offends anyone, I don't actually know if it is a he or a she and doesn't matter to me which.) As far as NH is concerned, I can't really keep up (haven't tried, tbh) with who helps me, but I hit everybody once a day (learned early on it was a great revenue source), hence my current pause on the event (can't give NH as quest wants for another several hours). As my neighborhood increases, my appreciation for the mobile interface grows.
Btw, one way I might guess level of activity is recognizing event-sourced buildings (especially multiples, though personally I keep one and disenchant the rest if I won multiples). Sometimes it can be misleading because many events (and event prizes) recur annually (or more often) so just because they won something doesn't mean they got it recently, could have been a year or two ago (I'm too new to know what pieces didn't exist two years ago, for example).