As E.T. said, Voldemort does not trust and leaves nothing to chance, but there are other reasons he had for believing that Snape was incapable of hiding information from him.
First, there are two branches of mind-manipulation in the Potterverse. The first is Occlumency and the second is Legilimency. Occlumency is purely defensive. Snape tells Harry in OotP chapter 24:
"Occlumency, Potter. The magical defense of the mind against external penetration. An obscure branch of magic, but a highly useful one."
Later in the same chapter, Snape and Harry have another conversation:
Snape looked back at him for a moment and then said contempuously, "Surely even you could have worked that out by now, Potter. The Dark Lord is highly skilled in Legilimency--"
"What's that? Sir?"
"It is the ability to extract feelings and memories from another person's mind--"
There are several reasons Voldemort would have known about Snape's abilities as an Occlumens:
- Voldemort knew he, personally, was a skilled Legilimens. He used the ability frequently to determine if those around him were lying to him, he used Legilimency and his connection to Harry to delve into Harry's mind and manipulate him, and it was probably unfathomable to him that anyone could be more skilled at Occlumency than he was at Legilimency.
- According to this timeline of Severus Snape at the Harry Potter Lexicon, Snape joined the Deatheaters when he was roughly 18/19 years old. Thus, if I interpret it correctly, he was a Deatheater before he was a member of the Order of the Phoenix. Voldemort knew the strengths and weaknesses of his followers--especially those Deatheaters who were higher up in the organization. Having an Occlumens at your disposal had the potential to be very useful, and while Voldemort himself could practice Legilimency, there is nothing to suggest that being skilled in one necessarily meant you were also skilled in the other. Had Snape joined the Deatheaters after aligning himself with the OotP, or if he'd still had serious ties to the Order or Hogwarts when he joined, I would agree that he might have tried to keep the secret, but at the time he really had nothing to hide.
- It would have been more dangerous for Snape to attempt to conceal his Occlumency from Voldemort than it would be for him to be up-front about it. Keeping such a skill secret from the Dark Lord would have appeared to be nothing less than treacherous, and we know what Voldemort did to followers whose loyalty appeared to waver.