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.

The techniques are discussed in this [blog post]. They were developed for the reverse engineering and (anti-)anti-tampering chapters of the OWASP Mobile Security Testing Guide. Note that this is only proof-of-concept code - it doesn't guarantee any kind of protection.