It isn't slang - it's simply a truncated form of 'Is that right?'. In casual conversation we often omit the beginning of sentences, providing the meaning is still clear.
For example, in informal situations, we'd often say 'Want a drink?' instead of 'Do you want a drink?'. The first two words are still 'there' in a grammatical sense, even though we don't say them. (NB Native speakers instinctively know which words can and can't be dropped, so omitting words at random is not something that foreign learners should try to do unless they're absolutely sure that this is possible).
What's more important than the structure, though, is what the policeman is implying by 'That right?'. It is most likely to be a rhetorical question. Especially if said with a raised eyebrow, smile and meaningful look, what he actually means is 'Do really expect me to believe that story?'. In other words, he suspects that the person is lying.