Luke 23.42-43:
And he said, “Jesus, remember me when you come into your kingdom,” (43) And he said to him, “Truly I say to you, today you will be with me in Paradise.” (ESV)
It has been noted by many that this thief on the cross received grace from Jesus with the most minimal of theology and a seemingly abbreviated request. But there is much that this thief expressed. The thief expressed in effect that Jesus dying on the cross was not the end of things for Jesus, but that Jesus would live and reign as King in his kingdom in heaven.
So this thief did the only thing he could: he asked the King of heaven to have mercy on him. He had previously admitted he was a sinner. And he was now in no position to do something at the Temple to make things right. So in asking Jesus to remember him when Jesus comes into his kingdom, it is reasonable to conclude that he believed in Jesus as the Christ and Lord of heaven.