I believe that having a deep connection with a soulmate does not automatically turn one into a psychic with the ability to specifically "sense" when that person's physical body has left this planet and transitioned to the next life. This would likely entail having a godlike power, which I doubt any of us possess.
My thoughts are that if we are not physically with our soulmate on a daily basis, we may just get an uneasy feeling or something compels us to Google that person out of the blue. Perhaps someone says something or we read an obituary that triggers a memory of our soulmate, which leads us to further investigate what happened to them.
Either way, we are only human - and while we may sometimes wish it weren't so, it appears that we are still very limited in the use of our "sixth sense" in that regard.