Here's how it works.
The US government runs a deficit. To cover it, the Treasury sells debt: bills (short-term, under a year), notes (2 to 10 years), and bonds (20 to 30 years). These are auctioned regularly. Participants submit bids specifying the yield they'll accept and the amount they want.
Once a month, the 10-year note goes to auction. Once a month, the 30-year bond. Every week, short-term bills. Four times a year, the Treasury publishes its refunding statement: how much it plans to issue, in what maturities, for the coming quarter.
The maturity mix is the first signal. When Treasury shifts issuance toward shorter maturities — more bills, fewer bonds — it often means confidence in long-end demand is low. You're borrowing short because you don't know if the long-end market will show up. That works until short-term rates rise or maturities pile up and the refinancing wall gets expensive.
