My last T would tell me his positive feelings for me directly. With this T, I see him interested and engaged and empathetic. Smiling at something I say and playful banter may be indicators.

It also feels like he cares when he holds the therapy frame and exercises good boundaries, such as not engaging in certain types of emails and texts with me.

I don't necessarily think extending a session and other ways of going outside of the frame as positive in a broader sense as those behaviors can come from a T trying to meet their own needs. Positive countertransference can interfere negatively with treatment.