Timeline to receive USCIS receipt number for L1A individual petition?

[VisaSeeker2015]

It is more than 20 days, since the L1A individual petition was received by USCIS. They have not sent the receipt number yet. How can I get the receipt number? I read that one can try to verify it on the back of encashed check. Are there any other options other than verifying the back of encashed check?

Any suggestions guidance is appreciated!!