Some random ideas for detecting Frida instrumentation from within a process:

• Scan all local TCP ports, sending a D-Bus message to each port to identify fridaserver.
• Scan text sections for a string found inside frida-gadget.so / frida-agent.so. File operations are implemented in ASM so prevent easy bypassing with libc function hooks.

These examples were developed to accompany a blog post. Note that copy/pasting this into your own code will not guarantee any meaningful protection.