When I have said it fairly loud with a head nod, I get quizzical looks and for the longest time I thought it was strange given they were wearing BYU gear.
So I tried for a month to keep saying it the way I normally do while flashing the the Y sign, I got the occasional quizzical looks, but most smiled and responded back positively.
So I finally dared myself to go all in and whenever I saw someone with BYU gear on I would put all of my energy and passion into it and really loud exclaim "Go Cougs!!!!!!" flashing the Y sign with both hands nodding my head smiling eagerly. Everyone I did this too got into it and smiled, laughed and replied Go Cougs back at me with several flashing the Y sign.
So my anecdotal study done over the course of 90 days told me that if you want a fellow BYU Cougar fan to respond to your "Go Cougs!!!" you need to really make it count if you are looking for 100% response results. Any other way and you are risking not getting the love back.