.OVA Format

.OVA - Open Virtual Appliance is a tar archive of a OVF - Open Virtualization Format "package". Simple archive must contain at least;

• Exactly one .OVF descriptor file, which is an XML file describing the packaged virtual machine's name, hardware requirements, references to the other files and other meta data.
• One or more disk images, (always?) in .VMDK format.

VirtualBox exports will also contain .MF manifest file, containing SHA256 checksums for included files. This is likely to be found in all archives, but should not be relied upon.

See also Open Virtualization Format Specification 1.1.0 from 2010.

Integrity Checking

Unfortunately, tar lacks features to make an integrity test on an archive file. Listing archive could be enough for some cases:

if ! tar tf archive.tar &> /dev/null; then
    echo "Problem!"
fi

But slow extraction will reveal issues that file listing cannot:

if ! tar xOf archive.tar &> /dev/null; then
    echo "Problem!"
fi

.OVF Descriptor

XML file - note: hardware information found at two distinct locations. Not sure which one to use yet.