Before opening, upload the .rar file to VirusTotal . This service will scan the file against over 70 different antivirus engines to see if it matches known malware signatures.
Identify exactly where the file came from. If it was an unsolicited email attachment, it is likely malicious.
Use a tool like 7-Zip or WinRAR to "Open Archive" rather than "Extract." This allows you to see the file names and extensions inside without actually running any code.