Were the iPhone 6s and 6s Plus big enough hits this holiday season for Apple (NASDAQ:AAPL) to outdo its blockbuster year-ago quarter? We'll find out on Tuesday.

iPhone 6s. Image source: Apple.

It would be difficult to overstate the importance of Apple's earnings report on Tuesday. As the company's first fiscal quarter of 2016, which is the fourth calendar quarter of 2015, the period captures a snippet of its business during the important holiday quarter. And investors are particularly honed in on this year's report as the company is up against its monstrous year-ago quarter in which iPhone unit sales soared 46% compared to the same period in the year before.

When the results are released, here are three metrics worth looking over.

iPhone sales
As always, the market will be watching Apple's reported iPhone sales closely. Now representing about 63% of revenue, this product segment's performance has a huge impact on Apple's overall results.

So, how many iPhones could Apple ship during Q1? On average, analysts are expecting Apple to deliver 76.6 million iPhones, according to a poll by Fortune. This represents 2.8% year-over-year growth in iPhone unit sales.

Investors can get a decent preview of Apple's expectations for Q1 iPhone sales by examining its expectations for the quarter's revenue. When Apple reported its fourth-quarter results, it said it expected first-quarter revenue to rise between 1.2% and 3.9% compared to the year-ago quarter. With iPhone sales representing the largest chunk of Apple's revenue, this guidance implies the company expected similar performance from its iPhone segment.

Earnings per share
With Apple's revenue growth likely to moderate to low single digits for the holiday quarter, it will be interesting to see what rate the company can grow its EPS.

Investors should expect EPS growth to track ahead of revenue and net income growth, driven by its significant share repurchase program. Consider the difference between Apple's fourth-quarter net income and EPS growth: On a 30.5% increase in net income between the fourth quarter of 2014 and the same period in 2015, its EPS leaped 38%.

Revenue in Apple's "other products" segment
When Apple reports results, many investors will likely look for clues about how well its 2015-launched smartwatch, the Apple Watch, is faring in the market.

Apple Watch. Image source: Apple.

Revenue for Apple Watch is buried in a segment it calls "other products," making it difficult to recognize exactly how well the product is performing. But based on some comments from management about its "other products" category during earnings calls, chances are that most of the year-over-year growth reported in the segment for Q1 can be attributed to the Apple Watch, giving investors a vague idea of how the new product may be performing.

In Apple's fourth quarter, "other products" revenue increased about 61%, or by about $1.2 billion. The company attributed the increase to the Apple Watch during the fourth-quarter earnings call. 

Estimating Apple Watch sales for the holiday quarter is difficult. The product is a bit of a wildcard, as it's the first time the device is sold during the quarter. But I'd say it's safe to predict sales of about $2 billion. Assuming Apple sees some growth in its "other products" segment outside of the Apple Watch, thanks to a recently revamped Apple TV, its "other products" revenue for the quarter will likely have to be about $5 billion if the company really does record around $2 billion in Apple Watch revenue during the quarter.

Investors can check in on Apple's results on its investor relations page after market close on Jan. 26. The company will also host a live earnings call at 5:00 p.m. ET.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.