When Does Fajr Start?
Question

What are the ways by which I can distinguish the true Fajr time? I do my best in the morning to know the real time of Fajr, in order to know the right time to pray and start fasting. I do this for me and my friends. 
As I live in China, Muslims here depend on the internet to know the times of different prayers. But it is not accurate, and thus they pray Fajr before its actual time. Will I be considered sinful if I could not be very accurate in estimating the right time for praying (I consider it when the darkness disappears and light starts to show)?
Praise be to Allah.We ask Allah to reward you with the best of rewards for your eagerness to learn the truth and do the right thing in your worship. We ask Allah, may He be exalted, to guide you and increase you by His grace in your love for knowledge and your keenness to learn. 
Types of Fajr
There are two dawns, the false dawn (al-fajr al-kadhib), which does not signal the beginning of the time for Fajr prayer or the time when the one who wants to fast should refrain from food, drink and intercourse; and the true dawn (al-fajr al-sadiq), which is when the time for Fajr prayer begins and the time when the one who wants to fast should refrain from food, drink and intercourse. 
This is what is referred to in the verse in which Allah, may He be exalted, says (interpretation of the meaning): “and eat and drink until the white thread (light) of dawn appears to you distinct from the black thread (darkness of night)” [al-Baqarah 2:187] 
The Prophet (blessings and peace of Allah be upon him) explained the difference between them in many hadeeths, some of which speak of the difference between them in terms of characteristics, and others speak of the difference between them in terms of rulings; and some hadeeths mention both characteristics and rulings. 
Difference between types of Fajr
The difference between the two dawns also appears clearly in the words of the Sahaabah and Tabi'een and the imams who came after them. 
Ibn Katheer (may Allah have mercy on him) said: 
“‘Abd al-Razzaq said: Ibn Jurayj told us that ‘Ata said: I heard Ibn 'Abbas say: There are two dawns: as for that which appears vertically in the sky, it does not make anything halal or haram; rather the dawn that appears along the mountaintops is that which makes it haram to drink. 
‘Ata said: If it appears vertically in the sky, that does not make it haram to drink for fasting or praying, and it does not affect Hajj. But if it spreads along the mountaintops, it becomes haram to drink for one who is fasting, and it means that Hajj has been missed. 
This is a saheeh isnad going back to Ibn 'Abbas and ‘Ata. Something similar was also narrated from more than one of the salaf (may Allah have mercy on them).” (Tafseer Ibn Katheer, 1/516)
Ibn Qudamah (may Allah have mercy on him) said: 
“To sum up, the time of dawn refers to the appearance of the second dawn according to consensus. That is indicated by the reports of the times of prayer. It is the white line that appears along the horizon; it is called the true dawn (al-fajr al-sadiq). 
As for the first dawn, it is the white line that appears vertically and not horizontally. It is not connected to any ruling and it is called the false dawn (al-fajr al-kadhib).” (Al-Mughni, 1/232) 
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: 
“The scholars stated that there are three differences between it (the false dawn) and the second (dawn): 
1- The first dawn is vertical and not horizontal, i.e., it extends from east to west (across the arc of the sky) whereas the second dawn extends from north to south (across the horizon).
2- The first dawn turns dark, i.e., this light appears for a short while, then it goes dark, whereas the second dawn does not turn dark, rather it increases in light and grows brighter.
3- The second dawn extends along the horizon with no darkness between it and the horizon, whereas the first dawn is separate from the horizon and there is darkness between it and the horizon.
Are any rulings connected to the first dawn? No shar‘i rulings whatsoever are connected to it, such as stopping eating when you want to fast, or the time for Fajr prayer beginning. The rulings are connected to the second dawn.” (Al-Sharh al-Mumti‘, 2/107, 108)
Are prayer timetables reliable?
With regard to the available prayer timetables, they are not a reliable source for finding out the time of Fajr prayer . The fact that these timetables are incorrect has been proven. 
What you should do is not rely on the timetables to find out the time of Fajr prayer. You have to look for the correct time on the basis of what we have mentioned of the differences between the false dawn and the true dawn. If you cannot look at the sky every day, then you can delay it after the time mentioned in the timetable, to be on the safe side. In our country this time differs from one city to another and from one season to another. So you can delay it half an hour, for example, to pray Fajr, but to be on the safe side you should stop eating and drinking before that. 
You can draw up a correct timetable to be used by those who come after you, after working out the time of the true dawn for a whole year, at various times. Perhaps you will attain the reward of correcting the Muslims’ acts of worship. 
Based on that, if possible you can work out the time of Fajr yourselves and follow that in prayer and fasting. If that is not possible, then you should not pray until you think it most likely that the time for the prayer has begun. 
When to stop eating in Ramadan
With regard to fasting , you can eat or drink until you are certain that dawn has broken, because Allah, may He be exalted, says (interpretation of the meaning): “and eat and drink until the white thread (light) of dawn appears to you distinct from the black thread (darkness of night)” [al-Baqarah 2:187].
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: 
“So long as he is not certain that dawn has broken, he may eat even if he is not sure, until he is certain.” (Fatawa al-Siyam, p. 299) 
And Allah knows best.