A valid point, Silenia. Timing and phrasing are very important when introducing skills. If the skill is going to be used shortly, the best three options (in my opinion) are:
to have another character present who already knows about said skill. This character would, at the appropriate time, presumably say something like, "Hey, you still practice that like you used to?" flashbacks. Show a scene where the character was using the skill and so would think of it and act without prompting. to have the main character (or another character, if you'd rather) see the skill being practiced in secret. This person can then bring it up as needed.
Of course, there's always the option of having the person seen/caught using a different skill that can lead to being asked about the skill needed.
"So, Char. B. I saw you picking pockets the other day. You, uh...pick locks, too, by any chance?"
Or, for that matter, wearing something that could be considered a tell-tale sign.
"Char. A, you wear arm guards. Do you use knives or am I lucky enough to be on the run with an archer who can pick our enemies off at a distance?"
"Char. C, you've got calluses like a sword master. If I steal a sword, would you fight with it?"