This verse (1 Samuel 20:7) is part of a conversation between Jonathan and David, where they are trying to determine Saul's true feelings towards David. David is hiding from Saul, who has become increasingly jealous and paranoid, and Jonathan, Saul's son and David's close friend, is trying to ascertain if Saul still intends to kill David.

Here's a breakdown of the verse's meaning:

"If he says, ‘It is well;’ your servant shall have peace:"
"He" refers to Saul.
"It is well" means that Saul is in a good mood and feels positively towards David. It indicates that Saul does not harbor hostile intentions.
"Your servant" refers to David, portraying him as loyal and subservient to Saul, his king.
"Shall have peace" means David can safely return and resume his normal position without fear of harm. If Saul says things are okay, David is safe.

"but if he is angry, then know that evil is determined by him."
"If he is angry" means that Saul shows anger or hostility towards David, even after Jonathan has pleaded on David's behalf.
"Evil is determined by him" means that Saul has made a firm decision to harm or kill David. He's no longer just in a bad mood; he has a specific, malevolent plan. The word "determined" implies a conscious, resolute intention.

In essence, the verse outlines a test that Jonathan will perform to gauge Saul's true intentions. Jonathan will observe Saul's reaction to a specific situation (David's absence) and interpret his response as a signal:

A positive response ("It is well") means David is safe.
A negative response (anger) means David is in grave danger and must flee.

The verse highlights the importance of communication and observation in assessing someone's true feelings and intentions, particularly in a situation fraught with danger and deception. It also reveals the volatile and unpredictable nature of Saul's character, leading to a desperate need for David to stay safe.