Apple (NASDAQ:AAPL) shocked investors earlier this month when it said revenue for its important holiday quarter would come in about $5 billion less than the low end of management's guidance range for the period. The news prompted shares to sell off about 10%, extending an already-steep decline for the stock in the final three months of 2018.

Of course, the surprising Jan. 2 update only included a handful of preliminary figures from the quarter. Thus, the fiscal first-quarter report on Jan. 29 may get extra scrutiny as investors mull over Apple's prospects. The preliminary revenue for the period suggests this will be its first quarter featuring a year-over-year revenue decline since the fiscal fourth quarter of 2016.

Ahead of these results, here's a preview of some important items worth watching.

iPhone XS and XS Max side by side.

iPhone XS and XS Max. Image source: Apple.

Earnings per share

We already know what to expect from Apple's revenue. In the preliminary update on the quarter, management said revenue should come in around $84 billion, down about 5% from the year-ago quarter. In addition, this is below its own guidance for first-quarter revenue of between $89 billion and $93 billion.

But what should investors expect from the bottom line? Despite declining revenue, Apple forecasts an all-time record for its earnings per share. This means EPS will at least be higher than its previous record of $3.89 from the year-ago quarter. 

Indeed, its EPS could be up by a double-digit percentage point compared to the year-ago period thanks to the company's share repurchase program, which has reduced shares from 5.12 billion in the year-ago quarter to about 4.77 billion in the first quarter of fiscal 2019, aiding EPS growth.

"Other products" revenue

With Apple noting in its preliminary update that lower than-than-expected revenue from iPhone, mostly in Greater China, accounted for virtually all of its revenue shortfall during the period, investors will be looking over the rest of Apple's product segments. The company will need to prove its other segments have the potential to help pick up some slack from its biggest product segment.

Namely, investors should check on the tech giant's two fastest-growing product segments: services and "other products." In its preliminary update, Apple noted that its services segment generated more than $10.8 billion of revenue (up 27% year over year), but didn't provide a revenue figure for its "other products" segment, which comprises sales of Apple TV, Apple Watch, AirPods, Beats products, iPod touch, and other accessories. However, management did say sales of its wearables devices -- Apple Watch, AirPods, and Beats products -- were up 50% year over year during the period. In this context, investors should look for growth in other products revenue to be approximately in line with the 31% year-over-year growth for the segment in its most recent quarter. 

Apple will report its first-quarter results after market close on Tuesday, Jan. 29.

Check out the latest Apple earnings call transcript.