Does the dictionary in the lose and found case belong to Tom?
失物招领箱:the lose and found case.
强调词典是否是Tom的,所以主语应该是词典（dictionary）
当然,你用楼下的说法：Is it Tom's dictionary 也是完全可疑的.意思都一样.
根据你的句式：
Is that your dictionary in the lose and found case,Tom?