What are some signs that a therapist may have poor boundaries with their clients?

Off the top of my ancient head:

Struggling with fantasies of deeper connections with clients, whether sexual or parental or other intense or intimate relationships beyond psychotherapy.

Failing to mention the client in supervision/consultation, out of fear the supervisor/consultant will advise return to ordinary healthy boundaries.

What is a common thought that keeps people up at night? Why do some people experience this?

Session-expressed curiosities about client details not relevant to the therapy.

Disclosing feelings, fantasies, and experiences to the client in ways not related to the work the client is engaged in.

These items can happen fleetingly, briefly, in any therapy, but if they’re frequent, it’s definitely time for the therapist to get some good, solid supervision/consultation.

Why am I so triggered and depressed over a minor thing?

Eager anticipation (or anxious anticipation) of the next session in ways that distract.

General Introduction to Boundaries from Panahi Counseling:

Routinely going over the time limit with certain patients, compromising the time for the next client.

Why do people keep saying they have evidence and have presented it that proves you're wrong even though they have none and haven't presented anything? Furthermore, what do they think you're wrong about?

Obsessing about clients outside of work hours.

Serious disappointment when the client cancels a session.

Frequent phoning or texting of clients to “check up on them and make sure they’re OK.”

Why are you a Muslim? Why is it Islam for you and not something else?

Sense of competition with persons who are important in the client’s life.