In order:
$game_party.actors[INDEX].skill_learn?(SKILL_ID)
Actor at INDEX learns the skill with SKILL_ID
$game_party.actors[INDEX].state?(STATE_ID)
Returns true/false if actor at INDEX is inflicted with the state with STATE_ID
$game_party.actors[INDEX].learn_skill(SKILL_ID)
$game_party.actors[INDEX].forget_skill(SKILL_ID)
Actor at INDEX learns/forgets skill with SKILL_ID
$game_party.item_number(ITEM_ID) > 0
Returns true/false if party has item in inventory with quantity greater than 0
If you are simply checking for all "true" you can simply do that. A block has to be supplied if you are checking conditions with this method.