It is not difficult to sight the new moon when it appears. Although it may be difficult to sight it in some places because there is too much smoke, it will not be difficult to sight it in other areas where the sky is clearer.
No matter how high smoke and buildings may be, they can never cover the moon like the clouds. The Prophet (peace and blessings of Allaah be upon him) has told us that if it is cloudy and we cannot see the new moon, then we must complete the month of Sha'baan and regard it as having thirty days.
So either the new moon can be seen through the smoke and buildings, in which case we should fast when it is seen;
Or it can be seen in another place where the sky is clearer, so we should fast when it is seen;
Or it is too cloudy and we cannot see it, so we should regard Sha'baan as having thirty days.
With regard to the size of the new moon when it appears, it is very small and cannot be seen except by one who has good eyesight.
And Allaah knows best.
Reproduced from Islam QA