Short answer: yes—there are telltale signs and simple steps.
Signs: sudden battery drain or data spikes, unknown apps or profiles, weird call/static, texts about SIM changes, duplicate device entries in Apple/Google account, unexpected bills or logins.
Steps (low-cost, low-intrusion):
- Check Apple ID/Google “devices” and remove unknown ones.
- Call your carrier and ask if another SIM/IMEI is registered.
- Change passwords from a different device and enable 2FA.
- Run a reputable scanner (Malwarebytes) and inspect installed profiles.
- If still concerned, factory-reset and set up as new (don’t restore a recent backup).
- Notify banks if you see suspicious activity.
If you want a simple monitoring option to watch for strange access, consider Spynger.