Here is a way to check some default cache mode. Although it's also kind of documentation, it's "online" and you get the default cache mode of your current QEMU version. As @dokotr5000 said, it depends on the version.
"qemu-img -h" and search for the
"cache" part. You will find something like the following.
'cache' is the cache mode used to write the output disk image, the valid
options are: 'none', 'writeback' (default, except for convert), 'writethrough',
'directsync' and 'unsafe' (default for convert)
The cache mode is associated with individual image files. And the default cache mode you get this way is the one to
rebase an image file, instead of the mode when you
create it. But at least it suggests the favorite default of current version of QEMU, and lets you make an educated guess.
I tried this on Ubunt 12.04, 14.04, and 16.04. All give
writeback as default.