There are some positive indications, which are as follows:
- A Feast for Crows and A Dance with Dragons were both published five years after the previous novel in the series. July 2016 will make it five years since the publication of A Dance with Dragons. This isn't exactly logical, but if it's happened twice in a row...
- George R.R. Martin has cancelled a whole load of appointments and appearances for this year to get the book done, indicating that he believes it's close enough that a few days here and there will make a big difference. He's also not written scripts for Seasons 5 or 6 of the TV show to give him more time to write the book.
- Last year, George reported that the rewrites on The Winds of Winter had been nothing like as arduous and difficult as those on Dragons.
- As noted above, the Spanish publishers are apparently expecting the book in 2016.
- George himself has said he thinks it is possible to get the book out before Season 6 is expected to start airing in April 2016.
- Compared to the excitement and regular updates we got when A Dance with Dragons was close to finished, there have been no comparable reports or updates for The Winds of Winter. GRRM suggested this might be the case, no updates until the book is delivered, but it's not a positive indication.
- Given that A Dance with Dragons had its cover art out in the wild and being discussed years before the book was done, it's odd that we haven't seen the same for Winter. That cover with the horn everyone's seen is fan art, by the way, not the real deal.
- GRRM recently retreated a little from his determination to get the last two books out before the TV show ends (probably in 2018), taking a more philosophical line that if the TV show overtakes, it overtakes.
- If the book is to come out in April 2016, it really needs to be finished before the end of this year, which is starting to loom large.
- For previous novels, George sent large chunks of the books to his editors a while before completion, allowing early editing of the novel whilst it was still being written. As of a few months ago, that hadn't happened (apart from some 360-odd manuscript pages, most of them held over from Dragons). This may mean a longer editing cycle for Winter will be required.
The Wars and Politics of Ice and Fire blog has an excellent "What we know" page about The Winds of Winter which is being updated with the latest information, which I recommend periodically checking.