Virustotal is an online service for scanning virus/malicious/suspicious files using several antivirus engines. Oh ya, did I forget to mention it's free too ? I'm not sure if it can detect packers or not.

Another good free online file scanning service will be Online malware scan by Jordi Bosveld. Besides scanning, it supports simple packers detection. I used to use this but lately it seems the server load is always extremely busy, not sure if the server got bombard/flood or there are really so many people using it.

I suggest scanning both packed and unpacked suspicious file that you want to scan in order to get the best result.