The most obvious sign is slow drainage or a complete backup. If you flush a toilet and the water doesn't go down, or if the sink fills up and doesn't drain, ice may be the culprit.
A dripping faucet or a partial clog can allow water to slowly freeze layer by layer until the pipe is fully blocked. frozen drain pipes
Complete Guide to Managing and Preventing Frozen Drain Pipes The most obvious sign is slow drainage or a complete backup
During extreme cold snaps (especially if temperatures are below 20°F/-6°C), allow a slow trickle of water to run from faucets connected to vulnerable pipes. Moving water is much harder to freeze than standing water. frozen drain pipes