An improved technique for verifying a license of a software product includes performing license checks with a server and passing to the server, as part of the license checks, a drifting digital code. The drifting code forms a particular drift pattern, which the server detects over the course of multiple license checks. The drift pattern is typically unique, or relatively unique, to the machine on which the software product is run, and changes in a manner that is difficult for malicious users to replicate on other machines. If a second copy of the software is installed, e.g., if the software is pirated, the second copy will produce a drifting code that has its own drift pattern, which differs from that of the initial copy. The server detects the duplicate copy by observing a divergence in the codes it receives during license checks.