Japan has the highest debt totals in the industrialized world. The Prime Minister of Japan was elected on a platform of re-inflating Japan's economy after 20 years of sideways movement. However, with the massive debt now in place, any move up in inflation will make the debt burdens intolerable.
First, the 10 year JGB yield shouldn't move above 2%. That's its 10-year peak, reached briefly in 2006.
Second, the major holders of JGBs should be watched, in order to see if they are starting to dump their JGB holdings. The notation is a little obscure; the chart lists JGB holdings for all the major players in trillions of yen. The biggest group are the Depository Corporations - i.e. bank-like entities, second are insurance companies, third are domestic non-financial entities (regular companies, households, etc), and fourth is the BOJ itself.
Third, its probably instructive to look at the overall size of the debt bubble in Japan, to get a sense as to why there is an issue to be concerned about. To have a sense of the magnitude of Japan's debt burden, we compare it with the debt burden in the US, which peaked in 2008.
Last, in order to keep things afloat, Japan must run a trade surplus. Here is the current balance of trade: