Edit: My reasoning for saying this is that these are the companies where great positions will be opening up and created very shortly. Also given that they've taken on funding, or are about to close, they've been put through their paces by the VC's, so you can have some confidence that they know what they're doing (This is also why i said Series A, not seed...seed is a real gamble)
This is much less based on metrics as the above response, but it is definitely important especially if you want to get into a company that can hit it big. And this is also a question (about which markets they plan to scale into) I think you can ask the company if/when they are interviewing you. Best of luck!
I think the best opportunities lie in companies that have already closed a Series A, which means they've picked up traction / validation, but still have a long way to grow.