al-Mayyit (Arabic: الميت) is the term to refer to the deceased in Islam. There are prescribed burial rites to be given upon the death of a Muslim, including the salat al-mayyit, or "prayer of the dead".
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
When you touch a cold dead body directly with out gloves, Ghusl of that touching becomes obligatory on you. If you can not perform Ghusl, then Tayammum becomes obligatory on you.
It is Mustahab (recommended) to wipe on the tummy of the dead body twice, one with first Ghusl with Sidr and second with the second Ghusl with Kafour except the dead woman if she was pregnant then it becomes Makrouh (disliked) to wipe on her tummy during Ghusl.
If they are not sure that the dead body was given Ghusl of Mayyit, they should preform Ghusl of touching the dead body (Ghusl of Mass-e- Mayyit) which is like any other Ghusl.