You can estimate it by last use or charge. The wallet will show "Latest Transactions". I make it a habit to add some funds before I go. This also means I don't top it up to max if doing a little forex guessing without a return trip within the next six months.

The Suica app also shows the stored fare history, but I haven't spotted an obvious Active or Inactive state. I could be missing it though since I don't read Japanese and my mobile suica is from the days when it needed to be added via app. Most visitors probably add it directly via Apple Wallet these days.

I think the HND Keikyu and monorail gate agents are probably a bit used to reactivating. I did notice JR trying out a "virtual" agent in the gate office at Takanawa station, plus subway IC gates don't always have a staffed gate at the entrance closest to you so I just add funds before going to avoid all that.

Same with my physical IC cards, actually. I add a bit of change to them at a fare machine or use them once in a combini to extend the last used date during a trip here and there. My travels are more like a domestic traveler, though, to some rather countryside spots where it might be unmanned paper only ticket stations (possibly ticket on one man train), eventually connected on a shinkansen, and then to an unstaffed IC based station. It still just eases my mind to have physical card and cash backups. True inaka updates are slow to arrive, if it ever makes sense financially.