Realty Income's (NYSE:O) revenue rose nearly 30% to $220 million during the first three months of the year, but that its net income fell by 25%. Does that make you nervous?

What if I told you that peer National Retail Properties (NYSE:NNN) saw its income jump by nearly 25%?

Would you be really nervous?

Well, it turns out the glance at these dividend giants may be deceiving, as real estate investment trusts like Realty Income and National Retail properties don't exactly depend on net income to pay out their massive dividends to shareholders.

Instead, there is one critical number to watch to gauge how well these companies are performing. The slideshow below tells you exactly what that is, why it matters, and how different a story it can tell.