"is 't 'm?" is short for "is het hem?" The meaning depends on the context. You can use it in sentences like the following: -- Is 't 'm gelukt om een huis te kopen? = did he manage to buy a house? I noticed in your comment to Chris that you came across it as a full sentence. That could work in a context like this. - "Iemand heeft net mijn tas gestolen." (Someone just stole my bag) - "Ik zie iemand rennen met een tas. Is 't 'm?" (I see someone running with a bag. Is that the person/the bag?)

Perhaps you should give us the complete sentence then we could give you a more definitive answer. Looks like a typo to me and really doesn’t make sense. It seems there are some missing letters.

The comment from Wanda is right. But I would not advise to use it this way. It's a bit street language and more used in writing and informal situations

Wanda's explanation is excellent. This contraction is something you'll hear quite often in (informal) conversations. It kind of sounds like "ist-uhm". Therefore you might find this in novels and such when a conversation is written down, but never in more formal texts.