You can use #skills at any time to consult your skills in all groups where you have more than 0%. Further, you can also use it to look up exactly which weapons and spells belong to each skill group. Frustrated vanilla and SLASH'EM players may not have known that athames belong to dagger group or that broad picks belong to the pick-axe group, but brass players can easily check. (It should also be noted that Nethack brass consolidates many skills, for instance grouping scimitars and sabers under the saber group.)
To enhance a skill, gain an experience level and a menu will prompt you to add 15% to any one skill--10% instead for skills at or over 50%, 5% for skills at or over 75%. Always choose a skill; unlike in vanilla, there is no limit where enhancing one skill might prevent you from enhancing another. In letting you choose a skill upon leveling, NetHack brass almost feels like ADOM, though ADOM is certainly more complex. (Note that if you lose a level and regain it, you may not claim the free skill points a second time.)
Skill percentages will also increase on their own with practice. At higher levels, this method becomes a faster way of raising skills than enhancing.
In source code Edit
The source code for brass still contains constants like P_BASIC and P_EXPERT, and you might apply some patch to weapon.c that still uses the skill_level_name function. Consult the table here. The constant P_BASIC contains 50, while skill_level_name will reckon anything from 50 to 74 to be Basic.
Note that P_ISRESTRICTED and P_UNSKILLED are both 0. A skill is only restricted if its max skill is zero. If the skill is 0 but not maxed, then it will still show in the skills menu, but only when leveling up. In brass, a skill unrestricted by a god is capped at P_MINIMUM or 25%.
Maximum skills Edit
To consult the maximum possible skills for your current role, do the #skills command; then press [w] for "show weapon capability" or [s] for "show spellcasting capability", for example:
For groups marked (++), the maximum expert 100%; for groups marked (+), the max is skilled 75%; other groups max at basic 50%.