SIGNS THAT RAMADĀN HAS BEEN ACCEPTED
Following good deeds with good is a sign that (this act) has been accepted, whether it is pertaining to Ramadān or other than it. If the Muslim is in a good state after Ramadān, performing many good deeds and righteous actions, then this is a proof of acceptance.
SIGNS THAT RAMADĀN HASN’T BEEN ACCEPTED
If the opposite is true, and he follows good with evil, and as soon as Ramadān is over he follows on from this with sins, heedlessness and turning away from the obedience of Allāh, then this is a proof that it hasn’t been accepted.
Every person knows their true situation after Ramadān. So let them look, if they are in a better state, then let them thank Allāh, as this shows (that it has been) accepted. If their state is worse, then let them repent to Allāh and seek the forgiveness of Allāh, as this is a proof that it hasn’t been accepted and it is a proof of negligence and carelessness.
Reference: Ithāf Ahlil Īmān bi Duroos Shahr Ar-Ramadān pg. 119-120 By Shaykh ‘Allāmah Sālih Al-Fawzān