The most important metric is excessive inflation, and the secondary one is exchange rates (though changes in that have mixed effects). These are empirical facts of the matter.
There's lots of theories about how this or that or the other thing might cause high inflation and/or devaluation. But when you look at the track record of these theories they are pretty horrible. QE1 and QE2 was supposed to lead to imminent hyperinflation. It didn't happen. The S&P downgrade was supposed to lead to yield spikes in treasuries (which in turn was supposed to lead to heavy inflation). It didn't happen. Deficit spending was supposed to lead to bond vigilantes coming out of the woodwork and cause catastrophic inflation. It didn't happen.
As a formal matter the government doesn't even need to issue bonds. Under this view (which broadly speaking goes by the name MMT) the government doesn't have a budget per se at all. When a bill comes due they can just create dollars out of thin air to pay it. When taxes or other fees are paid to the government they don't get stored anywhere to be used to pay bills, but instead cease to exist. Advocates of MMT suggest that the government tweak outflows and inflows not with the aim of balancing a non-existing budget but rather based on managing observed metrics (mainly inflation).
Critics claim that a move to such a system would cause hyperinflation. Who knows, they might even be right for once. The important take away is that the US government is not just a really big household, and US treasury bonds are not just like a credit card.
The reason we haven't seen inflation is that, despite all the printing, the money supply has not increased since 2008. This is because the velocity of money and the money multiplier have both decreased significantly since the financial crisis -- in essence, people are not borrowing or spending the way that they were in 2008. If people aren't spending, you aren't going to get inflation... until you do.
There's lots of theories about how this or that or the other thing might cause high inflation and/or devaluation. But when you look at the track record of these theories they are pretty horrible. QE1 and QE2 was supposed to lead to imminent hyperinflation. It didn't happen. The S&P downgrade was supposed to lead to yield spikes in treasuries (which in turn was supposed to lead to heavy inflation). It didn't happen. Deficit spending was supposed to lead to bond vigilantes coming out of the woodwork and cause catastrophic inflation. It didn't happen.
As a formal matter the government doesn't even need to issue bonds. Under this view (which broadly speaking goes by the name MMT) the government doesn't have a budget per se at all. When a bill comes due they can just create dollars out of thin air to pay it. When taxes or other fees are paid to the government they don't get stored anywhere to be used to pay bills, but instead cease to exist. Advocates of MMT suggest that the government tweak outflows and inflows not with the aim of balancing a non-existing budget but rather based on managing observed metrics (mainly inflation).
Critics claim that a move to such a system would cause hyperinflation. Who knows, they might even be right for once. The important take away is that the US government is not just a really big household, and US treasury bonds are not just like a credit card.