When Amazon.com (NASDAQ:AMZN) reports its second-quarter results after market close on Thursday, investors will be watching closely. Shares have surged recently, rising 58% year to date and 27% in the past three months alone. This has pushed Amazon's market capitalization from under $600 billion at the beginning of the year to about $900 billion today.

Investor optimism has been fueled by Amazon's expectation-crushing performance, featuring rapidly rising revenue and surging operating income. The company's relentless momentum means investors have high expectations going into Amazon's second-quarter earnings report this week.

While all this pressure will prompt investors and the news media to take a good look at the company's continued sales and operating-income growth when Amazon's results go live, there are other important areas investors will want to check on, too. Here are three metrics beyond these key financial figures that investors should look at:

Boxes in an Amazon fulfillment center

Image source: Amazon.com.

1. Subscription services revenue

One area where Amazon is seeing outsize growth that doesn't get enough attention is the company's subscription services revenue. According to Amazon's most recent earnings release, the revenue category includes Amazon Prime membership, as well as audiobook, e-book, digital video, digital music, and other subscription services (excluding Amazon Web Services). 

Amazon's subscription services revenue is soaring, highlighting how customers are becoming more invested in the Amazon ecosystem. Subscription services revenue in Amazon's most recent quarter soared 56% year over year when adjusting for currency changes. This is up significantly from 47% growth in Amazon's fourth quarter.

Amazon called out the enormous success of one of its subscription services, Amazon Music Unlimited, during its first-quarter earnings release. Subscriptions to the service increased more than 100% during the last six months alone.

2. Amazon Web Services revenue

Another area to watch will be Amazon's cloud services business, Amazon Web Services (AWS). The segment, which drives the bulk of Amazon's operating income, has seen its revenue growth accelerate for two quarters in a row.

AWS revenue in Q1 increased 48% year over year when adjusted to exclude currency changes. This is up from 44% growth in the fourth quarter of 2017 and 42% growth in both the second and third quarters of 2017.

With Microsoft's (NASDAQ:MSFT) Azure -- a competing cloud service -- gaining momentum, it will be interesting to see if Amazon's sharp growth continues unabated. Azure revenue continued to outpace AWS growth in Microsoft's just-reported fourth quarter of fiscal 2018, increasing 89% year over year. Further, Microsoft's broader commercial cloud-revenue category -- which includes revenue from Azure, Dynamics 365, and Office 365 commercial -- increased 53% year over year. 

3. International sales growth

Investors should also examine Amazon's sales by geography. While North America has been accelerating recently, international sales growth has been decelerating. Adjusted to exclude the impact of currency changes, international sales were up 21% year over year in Q1, down from 22% growth in the fourth quarter of 2017, and 28% growth in the third quarter of 2017.

Investors will see how Amazon is faring on these three metrics after market close on Thursday. In addition to reading Amazon's second-quarter earnings release, investors can tune in to a conference call to hear management discuss the quarter's results at 2:30 p.m. Pacific time.