[Ayantir]

if IsAchievementComplete(achievementId) and GetNextAchievementInLine(achievementId) ~= 0 then
    -- AchievementLine is present and everything is done.
elseif IsAchievementComplete(achievementId) and GetNextAchievementInLine(achievementId) ~= 0 and not IsAchievementComplete(GetNextAchievementInLine(achievementId)) then
    -- Achievement is done but next is not.
elseif not IsAchievementComplete(achievementId) and GetNextAchievementInLine(achievementId) ~= 0 and IsAchievementComplete(GetNextAchievementInLine(achievementId)) then
    -- Achievement is not done but previous is.
else
    -- Standard
end

It's a basic code, but you should consider all sublevels.