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.

It's not slang. We call it a colloquialism which means it is the ordinary, everyday informal language that people commonly use. It means 'Ís that true?'/'Really?'

I need more context to answer the question. How was the other lines of dialogue? The person could have meant it in a sarcastic or it can used in a condensing manner like how you speak to a child.

Hi Ensomih, Slang refers to words, phrases that are used very informally. They are often restricted in special contexts. Examples: Friend and Unfriend (being used in social media) My bad (meaning, I was bad) In your example, "that right" means "Is that right?". I am not sure if the next example is a slang. In Singapore, bus drivers who finish their last shift of the night display a sign that says "off service". What it means is "Not In Service". Hope this helps. Cheers, Lance