the holy Spirit provides it- there are excellent articles online about assurance of salvation- I don’t wish to give a sermon here on Fr-
Have you in your heart accepted Jesus Christ as Savior? If so, then you have God’s assurance of your salvation- Assurance is based on God assuring us we are saved IF we accept His son in the manner in which He tells us we must-
“Is your trust in the Christ who died for you and was raised for you? Do you trust in Christ’s death and resurrection on your behalf? If the answer is “yes”, then you have God’s own word and faithfulness that you have eternal life and will spend eternity with Christ.”
https://doctrine.org/assurance-of-salvation
The above link gives a very good rundown on assurance-
As well, The Fruit of hte Spirit is evident in His people- it is evidence that a person is saved- THis is another assurance from God-