How to find out a game servers are still active and being played online by people?

if a game is part of a franchise (like gears), only the most recent entry will likely be most populated. When it's standalone entry, like PUBG, it will usually remain populated until the game itself dies.

There are a few exceptions here, but this is a pretty good rule of thumb.

Notable exceptions would be Battlefield, as 3 and 4 are still very much populated, probably even more than the recent entries in the franchise. Because Battlefield covers different time periods, people play the time period they enjoy most. Another exception would be Halo, but this is a different exception. All of the old games are compiled into the Master Chief Collection. Even though these are all old games (the original being 20), it is still very active and you can find servers in any game or mode.