This isn't a skill check but is, instead, something you can just do
Take a free action to speak. Take a standard action to activate the command word activated magic item. Take a free action to speak again.
- Free action to speak: "I smell the flowers, pet the unicorns' horns, and taste the...
- Standard action to use the command word activated magic item: "...effervescent...
- Free action to speak: "...waters of the countryside's clearest rivers."
I mean, really, if you don't care about concealing the effect or that you used the item, caring only that you bury the command word among nonsense, swearing, and embarrassing personal revelations, that's fine. Shoving the command word into, for example, the middle of your lecture on beholder interior decorating only makes the foe who heard you deliver the lecture and who subsequently takes and tries to activate the item look foolish until the foe can tease from your lecture the correct individual command word. In the interim, such a ruse makes you look foolish and annoys your friends. (Nonetheless, despite all this, I've had players for whom such a petty revenge would taste very sweet, so it's cool.)
The concealed command word ruse likely won't last long after you've lost possession of the magic item. Using Items on Command Words says that
Sometimes the command word to activate an item is written right on the item. Occasionally, it might be hidden within a pattern or design engraved, carved, or built into the item, or the item might bear a clue to the command word.
So check the item before you start this plan, I guess? Then the section continues, saying
The Knowledge (arcana) and Knowledge (history) skills might be useful in helping to identify command words or deciphering clues regarding them. A successful check against DC 30 is needed to come up with the word itself.
So aside from using spells like detect magic, identify, analyze dweomer, commune, legend lore, vision, contact other plane, arcane sight or any of the other many ways to glean a command word, the GM may rule any dude getting a high enough result on the appropriate skill check determines the item's command word. In fact, the GM may rule that that dude needn't've even first heard the embarrassing sentence you shouted when you used the item, making all your efforts for naught, which is sad.