An Intelligence check is appropriate for this task. According to page 178 of the PHB, an Intelligence check can be used for a number of things, including:
- Estimate the value of a precious item
As far as what skill can be used on this check, or whether a skill can be used at all, I leave it up to my players to suggest a skill that might apply and justify it. This puts the onus on them to decide what they want to use, and it has the additional benefit of sometimes letting them use something that wouldn't have occurred to me.
For example, a conversation might go like this:
Player: Can I tell what this portrait is worth?
Me: Give me an Intelligence check.
Player: Can I apply my proficiency with painter's supplies to that?
Me: Sure, that sounds fair.
Note that an example of a very similar conversation appears on page 5 of the PHB:
Dungeon Master (DM): OK, one at a time. Phillip,
you’re looking at the gargoyles?
Phillip: Yeah. Is there any hint they might be creatures
and not decorations?
DM: Make an Intelligence check.
Phillip: Does my Investigation skill apply?
DM: Sure!
Phillip (rolling a d20): Ugh. Seven.
DM: They look like decorations to you. And Amy, Riva
is checking out the drawbridge?
My suggestions, coming from the other side of the fence where I (and some of the other players) feel that the DM plays a little too fast and loose with the rules, and makes changes to things that we think ought to be "canon" for the well-known world we are playing in:
1) Be willing to consider that the player may be right. Allow him to make a brief argument referencing the rules. Then make a ruling. Make a mental note of how often you rule against the player versus how often you change your mind and agree with him, and try (later, outside the session) to assess whether you're being particularly harsh and/or truly weakening one character's abilities relative to the others'.
2) Be firm if you still disagree with him. If he still disagrees with your ruling, tell him, "I need to ask you to go with the DM ruling for the moment and we can discuss it more later outside of game time, to figure out how we'll play this type of situation in the future."
2a) Try to offer the player another way to reach his objective. Say something like, "Look, the rules say that you give away your position if you attack from hiding. If you then, in full view of the enemy, duck behind the same tree, they are going to know where you are, even if you are so well hidden that they can't perceive you. Thus you do not get the advantages of being hidden in that case. Now if on your next turn you stealthily move to the next tree and hide there without being noticed, and then attack, that would be unexpected and give advantage."
3) Ask players not to use the Monster Manual at the table, and to avoid using metagame knowledge about monsters. That said, try not to mess with well-known monsters in a canonical setting without a really good story justification. If you're playing in a canonical setting, Mummies are going to be something that most adventurers will know the legends of, and the way that Mummies are described in this universe really does preclude a "good-aligned Mummy". If there's going to be a good-aligned Mummy, there should be a good story to go with that, to say how that happened contrary to the usual Mummy creation process, that the PCs have at least been given hints about. Otherwise, yeah, it's pretty appropriate for a PC to automatically kill any Mummy he comes across on sight. They will know the stories....
Note that the 5e MM does say (page 7 if need a reference for your rules lawyer):
The alignment specified in a monster's stat block is the default. Feel free to depart from it and change a monster's alignment to suit the needs of your campaign. If you want a good-aligned green dragon or an evil storm giant, there's nothing stopping you".
However, unless there is a good story behind the anomalous alignment, and your PCs have access to clues about that story, I think it would usually be better (and annoy your players less) if you either make up a new monster that isn't in the MM, or be clear that you are playing in a non-canonical setting and using monsters that don't match the descriptions in the MM. Even in a canonical setting, you can play variations on less-legendary monsters, but be clear (out of character) with your players that this is what you are doing. In all cases, allow the players relevant checks to recall some in-game, in-setting lore about the monster you are actually playing.
For example instead of just putting in a good-aligned Mummy you could say, "You see a medium-sized humanoid, wrapped in bandages. Make a religion check". Tell everyone with a low score that they think it's a Mummy. Tell whoever got the highest check, "Because of [some detail that they can perceive] you think this might not be a true Mummy but rather a Pseudo-Mummy. Pseudo-Mummies are created by a different process than True Mummies and in some cases can maintain their pre-death alignment." If you want, you can go into the process more, or you can just say that the character doesn't know any more than that. Now you have a good-aligned Mummy that your player shouldn't complain about.
4) Consider having a talk with the players about what game everyone wants to play. You have a conflict in play style with the "rules lawyer" player. Do the others also want to play "his" game, or do they prefer your approach? Can whoever is in the minority live with adjusting their expectations to what the group as a whole prefers? Can there be some compromise?
Best Answer
First of all, D&D 5 treats language as a binary - you know it with absolute fluency or you don't know it at all. You could stop right there and it would be fair.
If you want to go further and allow someone to recognise a written or spoken language they don't know you need to decide:
Which proficiency is appropriate?
Whatever one the player can make a reasonable case for
Depending on the language, I can see a case for:
However, I'm not the lawyer for the prosecution; it's not up to me to convince you. Let the player state their case: if they convince you it applies, if they don't it doesn't.