a person who watches or who keeps watch.

an analytic observer of trends, fashions, events, celebrities, or the like

Fashion watchers will have noted that pleats have become popular again.

a professional or experienced observer and analyst of political and historic trends and events, countries, or the like

China watchers in the State Department predict a change in that country's trade policy.

poll watcher.

noun

beholder , bystander , looker-on , observer , onlooker , spectator , shadow