On a more serious note, it’s important to remember that the point of code phrases like this is to allow members of a secret group to be able to recognize each other without being recognized by ousiders. That means they have to walk a thin line between being distinct enough for those in the know to recognize, while also being subtle enough to pass as casual conversation for those not in the know. Something like “never trust the hand you can see” sounds really cool and cryptic, which is exactly why it would be a poor code phrase. It’s just too obvious that it’s a code phrase, and even indirectly implies the name of the organization that employs it. On the other hand, you can’t go with something too casual like “nice weather we’re having” because the message’s intended recipient will have trouble recognizing it as distinct from ordinary small talk. This is why code phrases like this tend to have a call-and-response format; so if someone doesn’t give the correct response, you know they’re just making conversation. For example, in the film Quest of the Delta Knights, the eponymous Order of the Delta Knights recognize each other by the exchange:
“It’ll be a good day if it doesn’t rain.”
“It’s always a good day if it doesn’t rain.”
“Only if the sun isn’t too warm.”
This is an effective code, because the exchange appears totally innocuous to the casual observer. But the responses are in fact very specific, and not something a random bystander would be likely to give by accident. If you tell a random stranger, “It’ll be a good day if it doesn’t rain,” they’re more likely to respond with something like, “Sure, I guess,” or even “isn’t it always?” than the specific phrasing, “it’s always a good day if it doesn’t rain.” Likewise, if you’re on the receiving end, you would expect someone commenting on the weather to use more casual phrasing like, “It’s nice out today,” or “Sure hope it doesn’t rain.” So if you, as a member of the Order, hear someone say specifically, “It’ll be a good day if it doesn’t rain,” you can be relatively certain that they are prompting you to see if you’re in the know. “Only if the sun isn’t too warm” serves as an added layer of security for the recipient, just in case someone does coincidentally happen to use the exact wording “It’ll be a good day if it doesn’t rain.” If they don’t give the correct response to, “it’s always a good day if it doesn’t rain,” for example saying, “good point,” or “Not if it’s too hot out,” then you know it was a false positive.
So, something along those lines would probably be best. You don’t want to use that exact code, because if any of your players are big MST3K fans or just happen to have seen the film for some reason, it’ll be an instant giveaway. But a similar seemingly-casual but actually quite specific exchange will do nicely. Someone up thread suggested, “if it’s nailed down,” and “we take the nails too,” which is on the right track with the call-and-response, but is hard to pass off as casual conversation and sounds very much like talk of theft. Instead, consider something like:
“Did you hear the one about the halfling with the three-legged dog?”
“I have, but tell it to me anyway.”
“They say he came up a foot short.”
Or whatever. The joke format is nice because most people will not have heard such a joke. On the off-chance they have heard a joke like that, they probably won’t ask to hear it anyway, and not with that specific wording. Then by having a response that kind of sounds like it would be related to the joke, but isn’t actually very funny, you safeguard against actual jokes about halflings and three-legged dogs.