if[Not][UnitId]Has{Buff,Debuff}[{<,=,>}XX]=<buff1>[,<buff2>,...]
where XX refers to the number of buff/debuff applications,
e.g. ifTargetHasDebuff<5=sunder
and <buff> is the short code name of any of the predefined buffs.
To check for buff categories like DoTs, immobilization, etc. use ifUnitIs.
Also see ifUnitHasBuffTitle.
Legend
<item> is a placeholder and the entire string including <> should be replaced with the appropriate value. [text] means text is optional. {option1,option2,option3} means one of option1, option2 or option3 must be chosen. Batteries not included. UnitId can be any valid UnitId sequence as described in http://www.wowwiki.com/UnitId. Capitalization of UnitIds is not important. e.g. player, pet, target, targettarget. If the UnitId is optional, player is assumed.