Actually there are 2 ways to know that Darius is starting to have feelings for you.
1) He starts to refer to himself as "your lion" to the MC and never seems to realize he's doing this. This is a huge flag since it lets you know that your supportive friendship and sexual teasing him back is causing him to drop the walls he has around his heart.
2) Once he starts to refer to himself as the MC's "your lion", if you make choices that hurt him, his character cutout will change to reflex that fact you hurt him, even though he says nothing.
These 2 things help you to know your on the right track with him. Hopefully in the remaining days of his route we will be able to help Darius overcome the cause of the hurt that made him so overly protective of gettting hurt again (I have some suspicions as to what going on here).