You will just know. I have with all my horses. My friend actually found me my current boy and she also just knew and he Is actually my absolute perfect horse. When I tried him out I was so nervous I hardly did a thing on him but just knew. Had 2 friends ride him and confirm they would buy him if they were looking (we are all at same level/lik same thing etc) had him vetted and bought him. Before I collected himi literally could not stop staring at pics of him(still can't Sometimes) cos he's so handsome! Had been to see another whoi also liked and probably would have done the job but wasn't my type at all and I just didn't feel it and that's howi knew!
Personally, I have never "just known". I've had several horses and have always been pragmatic about it rather than falling madly in love. The one I have now I bought from someone I know who thought he'd suit me. I wasn't keen when I first saw him, wasn't keen when I watched them rode or when I rode him myself, until I jumped him and I enjoyed that. At no point during my two viewings (of him, not in total) did I think "OMG this horse and me are meant to be, we are soulmates". I did however think "we could work together, have fun, and when he grows up he will be a fab little horse". And he is. He's amazing and I'm delighted with him, but I'm not head over heels in love or anything like that.