I'm asking if it's normal for players to hide their dice rolls from the dm. I have a player who will purposely use the world's smallest dice to roll and then cover with his hands. A bit of information:He used to dm but doesn't any more. I've tried to play some games with him and I've noticed he always covers his dice unless he rolls a 20. Then he has to show everyone.  Is this normal? And should I stop believing his rolls.

No

It's not normal. There's usually one person that always sees the rolls - the DM. Sometimes the rolls might be hidden from the player, but usually never from the DM.

Personally, I'd insist on the rolls being open. I don't necessarily have to see it as DM, so long as other people can verify it, then it's fine, but there can be no attempt to hide it (I trust my players that if dice roll somewhere where they're hidden by accident, I just trust their report).

The DM can roll openly or hidden, but players should be open. I'd insist on that. 

As an aside, I roll hidden as DM. I do show players highly improbable results though - the other day I rolled something in the roughly one in tens of thousands range and just had to show my players! Partly because it was funny, partly because I didn't want them to think I was cheating.

As mentioned, Player's should not hide their rolls from the DM - only the DM can choose to hide their rolls.

Talk to the player. Tell them they shouldn't hide their rolls as it will make everyone else in the game think they are cheating. If they refuse, you can say as the DM 'fine, from now on, I'll be making all your rolls for you'.

Nah. Players are obligated to roll open. A player who hides all rolls except 20s is a behavior that should be adressed. Only the DM can make hidden rolls.

In a game I ran, I had a player that would try and reroll tons of low rolls when they were cocked and not do the same when the dice landed on a higher number. Eventually, I just said that everyone had to roll on the table and that anything that went off it automatically didn't count.

The situation here may not be as obvious, but you the DM should see your players rolls and this person is honestly super suspicious. At some tables, trustworthy players don't always have to prove their rolls were what they were, though that can lead to a slippery slope with those who are untrustworthy because that rule would then have to be revoked. Anyways though, I would like to parrot the advice of others that rolling in the open is mandatory. I would tell the player something along the lines of: "I doubt you're cheating, but this sets a bad precedent and I need to be sure your rolls are what they say you are."

Some people - especially cheaters - can't stand to be accused of even the possibility of not always playing fair. If the person in question puts up a massive argument over this, you might not want to continue playing D&D with them.

The players show their rolls.  I show my rolls as the DM as well to up player tension.  

Tell the player to show his rolls or he can leave.  If he's using purposely ridiculously small dice, ask him to use normal sized dice or he can leave.  

Merely not showing rolls would be a normal way to go. When doing DnD over Discord with friends we're not constantly sharing screen to show what we rolled. And we have enough trust with each other that we're not fudging the results. On the flip side we'll sometimes voluntarily show our rolls because we're amazed at our good luck.

But I agree with that others that hiding the rolls, when asked, is weird.

Quote from VitusW >>

Merely not showing rolls would be a normal way to go. When doing DnD over Discord with friends we're not constantly sharing screen to show what we rolled. And we have enough trust with each other that we're not fudging the results. On the flip side we'll sometimes voluntarily show our rolls because we're amazed at our good luck.

But I agree with that others that hiding the rolls, when asked, is weird.

 That's more of an adaptation to the fact that it's awkward to show rolls. There's a difference between not going out of my way, and taking up valuable playing time, to publicise my rolls, and going out of my way to hide them from everyone else.

As DM, I don't see maybe 60% of player rolls, but most every player rolls will be seen by someone. I trust players so long as they're not trying to hide their rolls.

Some players use personal rolls to help them make character decisions they're waffling on, and will hide those rolls from other players or potentially even the DM since it's a way for them to address indecisiveness on their part, not a formal table roll, and they don't want to make a big deal out of whatever they rolled.  It's a roleplaying aid for them, not a roll to get excited or upset about.

If the DM called for the roll, however?  There is absolutely no excuse for trying to hide it from the DM.  As Linklite said, there's a difference between "this is awkard to show off, but I promise I'm being good" and this guy actively moving to cover his dice at an in-person table.  The former is just Internet shenanigans; the latter is deeply suspicious and generally held to be unacceptable.  There really is no excuse for it.  You don't have to grill him like a panini or be hostile or aggressive about it, but as a DM I wouldn't allow someone to consistently make rolls I didn't get to see.  Especially if they had a history of being sus already.

Quote from Yurei1453 >>

Some players use personal rolls to help them make character decisions they're waffling on, and will hide those rolls from other players or potentially even the DM since it's a way for them to address indecisiveness on their part, not a formal table roll, and they don't want to make a big deal out of whatever they rolled.

If you're rolling dice that you aren't going to report to anyone else, it hardly matters if you're being honest. Dice that matter to anyone but you should be rolled publicly where practical.

Generally, the more a player tries to hide their dice rolls, the more likely it is that they're cheating.

I would talk to the player and tell him to stop with this behavior.

If he does it again call him out on it and ask to see the roll - however it can be done depending on whether you play in person or online.

If he blatantly refuses to show his rolls then I would tell him he failed his skill, missed his target, and found a rat turd in his rations.

"I did hit it!"

"Show me the roll."