i will say it is easy to know when BTC has become strong there will be 2 cases happen, one is Altcoins fall to make BTC stronger and become the one and only, Two is Altcoins will rise along with BTC and bring enitre market up, i personally think the second way is possible because Altcoins is very strong now and even more advanced when compared to an old and slow BTC, so BTC should be used in trading and Altcoins can be used in daily life payments.