You can use the functions ptrace, ptraceError, ptraceIfFalse, ptraceIfTrue (from Plutarch.Trace or Plutarch.Prelude) for tracing. These behave similarly to the ones you're used to from PlutusTx.
If you have the development flag for plutarch turned on - you'll see the trace messages appear in the trace log during script evaluation. When not in development mode - these functions basically do nothing.