Persian will have three dots in a triangle above a single upward stroke or below the line. Arabic only has the three dot combo above the script on a multiple upward stroke grouping (sometimes a flat line between upstrokes).
The same is true for Urdu as well, so if you want to distinguish Urdu from Persian: look for a backward moving (i.e. towards the right) horizontal stroke at the end of a word. This stroke will always run under the preceding letters of the word, except that some dots of the preceding letters may be moved beneath the stroke in order to avoid collision.