How can I tell if software solutions to my labs are posted online?
You can't tell if solutions to your problem sets are posted online. The internet is HUGE and some things are hidden behind paywalls or other login forms.

But what you can do is:


design your own exercises and modify them enough each year to make it harder to re-use solutions.
quiz students about their solutions to (easily) find out if they did it themselves or at least understand the code.
wish your students good luck with their exams, where they will not have internet access.