On Thursday, Apple (NASDAQ:AAPL) began selling the iPhone SE, its first new 4-inch iPhone since 2013. The new model is very similar to the iPhone 6s in terms of features and performance, except that it is missing Apple's new 3D Touch technology. Yet it is much cheaper, with a starting (unsubsidized) price of $399, compared to $649 for the iPhone 6s.

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The iPhone SE hit the market on Thursday. Image source: Apple.

The iPhone SE is the cheapest iPhone that Apple has ever launched. It's also the first time the company has launched a new, lower-cost model in between flagship iPhone launches. As a result, it won't be straightforward to determine whether the iPhone SE is a "success" or not.

Growing in the mid-range market
Ever since Apple launched the iPhone 6 and iPhone 6 Plus in late 2014, its 4-inch iPhones have been positioned at the bottom of its product lineup, targeting the mid-range market. At the iPhone SE launch event, Apple noted that it sold 30 million 4-inch iPhones during 2015.

Apple achieved that performance even though its newest 4-inch model -- the iPhone 5s -- was already 15 months old at the beginning of 2015. The company hopes that launching an affordable 4-inch phone with up-to-date specs will spark upgrades among iPhone fans who prefer the smaller models, while also growing Apple's market share among price-sensitive smartphone buyers.

Don't expect lines or price premiums
Clearly, Apple ought to be able to sell more than 30 million 4-inch phones this year with a brand-new offering in the market. Unfortunately, it doesn't usually report sales by individual model, so we aren't likely to find out directly how the iPhone SE is doing relative to that milestone.

Also, many of the usual metrics that pundits use to judge iPhone sales don't make much sense for the iPhone SE. For example, some news outlets reported on the lack of lines for the iPhone SE launch.

However, the vast majority of the market has moved to larger devices. Apple may have sold 30 million 4-inch iPhones last year, but that also means it sold about 200 million larger iPhones! Furthermore, the technology in the iPhone SE isn't new. People who want to get the latest features as soon as they are available -- i.e., those who would normally line up on the first day -- probably bought the iPhone 6s.

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Apple's larger iPhones outsold the iPhone 5s and iPhone 5c by a mile in 2015. Image source: Apple.

Meanwhile, CNBC reported that scalpers in China didn't expect to be able to sell the iPhone SE at a premium, whereas other iPhones have sold for about $300 above list price initially.

This isn't surprising (or alarming), either. Most people in China prefer larger-screen phones. The main point the iPhone SE has going for it in China is that it is more affordable. Clearly, price-sensitive consumers wouldn't pay $300 extra to get the iPhone SE a few weeks early.

A small pool of early adopters
Based on the iPhone SE's target markets, it's not likely to see a flood of early sales. But there's certainly a contingent of iPhone 5/iPhone 5s users who have been waiting for a new 4-inch iPhone to upgrade.

To get a sense of how big this demand pool is, investors can monitor the wait times on Apple's online store. As of Thursday evening, within the U.S., most iPhone SE versions were available to ship in 3 to 5 days or 7 to 10 days. Many U.S. Apple Store locations had good stock of the iPhone SE, but a significant minority of stores were completely sold out.

Given that the iPhone SE should be fairly easy to build, production constraints shouldn't be a major factor limiting supply. (However, Apple may have placed very conservative component orders initially.) If it takes more than a couple of weeks for lead times to shrink to 24 hours, it would suggest that demand is quite robust.

Demand should remain steady
Due to its low price, the iPhone SE will appeal to price-sensitive consumers who are more likely to buy a new phone when their old one stops working, rather than on a set one-year or two-year upgrade cycle. This means that iPhone SE sales might not peak quickly and decline after a few months, as has been typical for other iPhone models.

We'll get some sense of how the iPhone SE is doing in a few weeks, when Apple provides its guidance for the third fiscal quarter. Apple is likely to post a double-digit iPhone sales decline in the second fiscal quarter. If it can get back to the flat line (or close) in Q3, it would be largely thanks to the iPhone SE.

iPhone SE sales may also hold up well during the fourth fiscal quarter, because Apple isn't likely to put out a new 4-inch model in September. By contrast, sales of the iPhone 6s and iPhone 6s Plus will probably slow even further over the summer as would-be buyers hold out for the iPhone 7. If Apple can get back to iPhone sales growth in Q4, it would also suggest that the iPhone SE was performing well.

Of course, it would be much easier to gauge the iPhone SE's success if Tim Cook provides more details of how it is selling during Apple's upcoming earnings calls. Whether he will oblige remains to be seen.

Adam Levine-Weinberg is long January 2017 $85 calls on Apple. The Motley Fool owns shares of and recommends Apple. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.