From the above website
Users should also avoid buying unbranded or ultra-cheap devices from unknown sellers. Stick to manufacturers that offer ongoing firmware support and publish clear security documentation.
If your device feels sluggish, heats up unexpectedly, or shows signs of unusual internet activity even when idle, it might be infected. Other red flags include Google Play Protect being disabled or missing entirely, unfamiliar apps appearing on their own, or the device being from an off-brand manufacturer without verified firmware. These signs could point to malware like BADBOX 2.0 running silently in the background.