How to recognize safe queries in tuple relational calculus?
I attended a course on database theory some time ago, but I’m a bit rusty on the topic, so take what follows with a grain of salt.

If by “safe” you mean “domain independent”, then I think you’re out of luck: domain independence is undecidable, as unsatisfiability of relational calculus queries reduces to it.

I believe you can find the details on the slides for the above-mentioned course, and in the book Foundations of Databases by Abiteboul, Hull, and Vianu.