Markel Corporation (NYSE:MKL) is set to release first-quarter 2015 earnings soon, and it's time for investors to consider what to expect.

Analysts, on average, predict that Markel's revenue will climb 5.3% year over year to $1.31 billion, which should result in net income of $5.88 per share. But especially in the case of financial holding companies such as Markel, investors are wise to dig deeper to understand what drives those results.

Here are four things I'll be watching closely when Markel's report hits the wires.

1. Book value per share
First, arguably the most important metric for gauging Markel's success is growth (or lack thereof) in book value per share. In fact, much in the same way Warren Buffett begins each of his annual letters to Berkshire Hathaway shareholders with a table outlining historical growth in book value per share, Markel regularly kicks off each of its quarterly reports by immediately noting the metric. 

For reference, last quarter Markel capped a solid 2014 by increasing book value per share 14% year over year to $543.96. And keeping in mind Markel management is compensated in part based on the five-year-rolling average of growth in book value per share, its compound annual growth over the previous five years was also 14%. Ideally, that trend will continue going forward.

2. Underwriting results
Next, look for Markel to provide color on the underwriting profitability of its core insurance businesses.

This will include individual percentages for combined ratios of Markel's U.S. insurance segment, its international insurance segment, and its reinsurance segment -- the last of which largely originated from Markel's acquisition of fellow insurer Alterra in mid-2013. Markel will also include a consolidated combined ratio to measure the collective success of its insurance operations overall.

Markel's consolidated combined ratio was 95% for all of last year, which means it earned $5 for every $100 in premiums it wrote. That included combined ratios of 95% at U.S. insurance, 93% from international insurance, and 96% from the reinsurance segment.

3. Investment income
Next, listen for details on Markel's investment income. This is where things get really interesting for shareholders looking to cash in on the long-term investing expertise of Markel President and Chief Investment Officer Tom Gayner. 

Updates here will include the dollar amount of total invested assets, as well as a breakdown of how much of that total consists of equity securities. At year-end 2014, total invested assets were $18.6 billion, including $4.1 billion (or 22% of the total) in equities.

Investors will also be given stats on how much investment income rose or fell over the same year-ago period. Last quarter, net investment income increased 5.2% to $93.25 million.

Finally, Markel will offer figures on its net unrealized gains, which last year increased by a whopping $661.7 million to $1.8 billion. Keep in mind if Markel were to take profits on these positions -- which were quite literally purchased with the intent of owning them forever -- it would incur an unnecessary tax liability of over $500 million. By continuing to hold Markel's equities investments for the long haul without selling, Tom Gayner is happy to let the magic of compounding returns do its work.

4. Supplementary businesses
Finally, watch for updates on Markel's non-insurance businesses, namely those acquired and now operating under the ever-growing wings of Markel Ventures. 

Here you'll see figures on consolidated revenue, (up 22.8% last quarter to $253.3 million), split between manufacturing and non-manufacturing operations, as well as a much smaller "other" category. Markel will also outline Markel Ventures' net income ($9.9 million last quarter) and adjusted EBITDA ($24.8 million in Q4).

And perhaps most intriguing, Markel will detail both the progress of previous acquisitions and any newly acquired businesses over the course of the quarter. New acquisition activity has been quiet of late, most recently including the purchases of a majority stake in auto transport trailer manufacturer Cottrell, and two baking equipment makers last July and August, respectively.

To be fair -- and keeping in mind Cottrell was Markel Ventures' largest purchase yet at $130 million -- Markel is notoriously prudent in its criteria for acquiring new companies. Markel Ventures even insists on its website: "Our focus is long term ... really long term. We do not sell our businesses, nor do we rely on the use of acquisition-related debt."

Make no mistake: That's great for long-term investors willing to watch as Markel's patient approach continues to create shareholder value over the years. 

In the meantime, though the three items above are hardly all-inclusive, they should serve as a solid basis to help investors better understand Markel's impending quarterly report.