How can you tell if thoughts are coming from your sinful side or Satan?  

Satan cannot enter into a blood-bought, Holy Spirit sealed Christian.  He can mess with us on the outside, but he cannot enter in, or put thoughts into our minds.  However, he can tempt us with things that will cause us to think bad thoughts. But we always have a free-will choice.

Whenever a bad thought begins to enter our minds, our choice should always be to turn our thoughts away from the bad and think on “whatsoever things are true, whatsoever things are honest, whatsoever things are just, whatsoever things are pure, whatsoever things are lovely, whatsoever things are of good report…” (Phil 4:8).

We also want to stop bad thoughts before they arise by renewing our minds every day (Ro 12:2), by remembering the Lord’s promises and His faithfulness, and by keeping our minds focused on Jesus (Heb 12:2).

With renewed minds, and by the power of the Holy Spirit, we are able to conquer any temptation.

Promises About the Mind: Col 3:2, Matt 22:37, Prov 27:12, Ro 12:3
Read About Conquering Sin In Our Sanctification Journey: What you need to know about Sanctification & Glorification |  How Do I Fight Impure Thoughts?  | Will impure thoughts condemn me? How can I fight them?  | Is My Heart Deceitful Above All Things? (Jer 17:9) Don’t I Have a New Heart?   |  Sin, Iniquity, and Transgression? How are they different?