Experiencing significant battery drain on your phone could be a sign of malware, especially if an unfamiliar app is consuming a large portion of the battery.
If your phone becomes excessively warm even when not in use, it may indicate malware forcing the processor to work excessively.
Sudden spikes in data usage without explanation could be a result of malware uploading data without your knowledge, so monitoring data usage is important.
Unexpected behavior like apps launching on their own, random restarts, or strange messages may indicate malware presence on your phone.