All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
When a believer dies, he receives good praise, is mentioned with his virtuous characteristics, supplication is made for him, a funeral prayer is offered over him and good is expected for him from Allaah The Almighty. In confirmation, the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, said: “Paradise will be assured for him whom you praise with good.” [Muslim] He, sallallaahu ‘alayhi wa sallam, further said: “The trial (i.e. disease) continues to purify the believer till he meets Allaah with no misdeed.” [Ahmad and At-Tirmithi]
But a martyr should be one of those enumerated in the Hadeeths. Perhaps Allaah The Almighty may Join him with them out of His mercy and bounty, depending on His saying (what means): {And excellent are those as companions. That is the bounty from Allaah.} [Quran 4: 69-70]
That his body is kept from decomposing after his death is neither supported nor refuted by any proof, which means that it is probable.
As for the signs of one’s death on faith, some are mentioned by scholars including the brightness of his face, the softness of his organs, the lightness of his dead body, the easiness of his burial and transportation, and so on. These are all good signs by which good is expected for him.
Allaah Knows best.