Yes. They know when we visit their graves and feel happy because of our visiting their graves and reciting Quran and praying for them. They hear us when we talk to them. The Hadeeth says: Talk to the dead people in their graves and raise your voice talking to them and tell them: You went before us and we are following you soon. يا أهل الديار من المؤمنين والمسلمين، أنتم لنا فَرَطٌ وإنّا بِكُم إن شاء الله وشيكاً لاحِقون
It is highly recommended to say when you visit the graves of the Muslims: Peace upon you, O people of Laa Ilaaha Illallah السلام عليكم يا أهل لا إله إلا الله.
How did find Laa Ilaaha Illallah كيفَ وَجَدتُم قَولَ لا إلهَ إلّا الله