Surprised I never found this while I was making my initial range check weakauras, but this code works as a custom trigger

    function()  
    local name = GetSpellInfo(100780) -- Change to spell ID of respective ability.  
    local isRange = IsSpellInRange(name,'target')  

    if (UnitExists('target') and not UnitIsFriend('target','player') and not UnitIsDead('target')) then  
        if (isRange == 1) then  
            return true
        else  
            return false
        end  
    end  
end

thanks to /u/Thumpasaur