The line is so often said "you'll know when the time has come". When my first pig, Charlie, was ill with a kidney tumour (diagnosed by xray and full examination), I spent most of those nine months that he lived past diagnosis wondering how I would know if it was time. He had a few days where he was hunched up a little, obviously uncomfortable - but still getting up once in a while and always taking any food he was offered, not in enough pain to stop him eating! Those days are the ones I worried the most about whether it was his time, how would I know whether it was too much for him, whether the tumour was just causing too much pressure internally and causing him more discomfort than was fair. But he always picked up a day later...and I slowly followed suit!
There is simply no way to describe how you know when it's time...you just know. Not a useful piece of advice but there's no other way to say it. It hit me in my heart, my head and my stomach when I realised it really was Charlie's time, quite aside from the symptoms that had developed overnight the night before he was PTS, you don't miss that feeling, it is something quite unlike anything else.
I sincerely hope you don't have to experience this, but as Joanne said, the fact that you are questioning it so much, and the fact that Mirtle is pottering about and eating, it's not her time, not just yet.