A practical first step is to search the relevant trademark registry. In the U.S., you can use the USPTO’s Trademark Electronic Search System (TESS) to look for the building name or related design marks; landmark buildings such as the Transamerica Pyramid have appeared there.

That said, trademark questions are highly jurisdiction-specific, and simply finding lots of photos online does not prove unrestricted commercial use is allowed. If you plan to make commercial use of a photo centered on a specific landmark, the safest approach is to contact the building owner or rights holder for permission.

For anything important or high-value, consult a lawyer rather than relying on photography forums, since trademark and architectural-image issues vary by country and by intended use.