Nothing. The only way to track regular mail is to see if anything happens as a result of it. IRCC will either let you know when the sponsor's been approved, or they'll send it back as incomplete or problematic.
You can 'hack' it slightly if you've had previous dealings with IRCC, so you have a UCI (ie, a prior work permit, study permit, PR application, etc). Use your UCI to try logging onto ECAS - you won't be able to at first, but you'll be able to logon once they start working on it, which can be weeks before sponsor approval.