Nothing very definitive but some indications are given in the Bhavisya Purana:
Then Bali and Kalki Devi approached Narayana in the form of Vamana who
in turn enabled a Brahmana called Kama Sharma and his wife Devahuti on
the banks of Yamuna River; he blessed that this couple would give
birth to Bhoga Simha and Keli Simha. These two sons would stay in a
Kreedavati Nagar and would carry out the wishes of Kali Yuga Devi,
especially in the task of wide-spread Varna-sankara or destroying the
Rules of Varnashrama.
Over two thousand years, the established Regulations made by Lord
Brahma and the successive Manus would get thinner and thinner and by
the Second Paada of Kali Yuga, Kali Devata would be happy to witness
that the old Vedic values would be obliterated, that the Daityamaya
human beings (fully soaked in Daitya activities) would be of
two-and-half feet height, that their life span would be forty years
maximum (as against hundred years now) and that they would be free
like birds without any Karmic regulations!
What happens even after that ? i.e in the last remaining quarters of Kali Yuga are also given in the same chapter:
At the end of the Kali's second half there would neither by the
institutions of marriages, nor Kingships, nor any social reformer and
not even a Karma Karta! The World would be full of the progeny of
Bhogi Simha and Keli Simha and this kind of a situation devoid of
customs and social norms woud prevail for one and quarter lakh years!!
In the Third Quarter of Kali Yuga, the average age of human beings
would be twenty six years maximum;They would be too lustful and
produce too many children and resort to affairs with co-males and
animals!....... In the fourth phase of Kali Yuga the maximum age of
humans would not exceed tewnty years and live like water-beasts and
animals; hells like Tamistra and worse kinds of frightening Places of
Retribution would be over-populated
So.basically, when we find human beings are having heights of around two and a half feet and when their life span on average is not more than 40 years we can assume that the first pada of Kali is over.