NAAP site to test Flash installation:
http://astro.unl.edu/naap/motion1/cec_both.html
 
Adobe site to test Flash installation:
https://helpx.adobe.com/flash-player.html
 
3rd-party site to test Flash installation:
http://isflashinstalled.com/
 
3rd-party site to test Flash installation:
https://www.codegeek.net/services/resources/flash-player-test-page/
 
Adobe site to install Flash (beware unneeded/optional/suggested components):
https://get.adobe.com/flashplayer/