When it comes to determining if an ISTJ (Introverted, Sensing, Thinking, Judging) is flirting, it can be a bit challenging as they tend to have a reserved and practical nature. However, there are a few signs that can indicate when an ISTJ is interested in someone romantically. Here is a detailed description of how you can identify if an ISTJ is flirting:

1. Increased attention: ISTJs are typically focused and observant individuals. If an ISTJ is flirting, they may display heightened attentiveness towards the person they are interested in. They will pay close attention to their conversations, body language, and even remember small details about them.

2. Engaging in small talk: ISTJs are not known for being big talkers, especially when it comes to casual conversations. However, if an ISTJ is flirting, they may initiate small talk or engage in more light-hearted conversations. They may ask questions about your interests, hobbies, or personal life to get to know you better.

3. Displaying subtle body language cues: ISTJs may not be the most expressive individuals, but they can still exhibit subtle body language cues when flirting. They may maintain eye contact for longer periods, smile more often, or lean in slightly towards the person they are interested in. These gestures indicate their desire to connect on a deeper level.

4. Offering practical help: ISTJs are known for their practicality and reliability. If an ISTJ is flirting, they may offer practical help or assistance to the person they are interested in. They might offer to fix something, help with a task, or provide advice on a problem. This is their way of showing care and support.

5. Sharing personal information: ISTJs are generally private individuals, but when they are interested in someone, they may open up and share personal information about themselves. They might talk about their goals, dreams, or past experiences, allowing the other person to get to know them better.

6. Initiating one-on-one activities: ISTJs prefer intimate and meaningful interactions rather than large social gatherings. If an ISTJ is flirting, they may suggest or initiate one-on-one activities such as going for a walk, having coffee, or watching a movie together. They value quality time and use these opportunities to build a deeper connection.

7. Showing loyalty and commitment: ISTJs are known for their loyalty and commitment once they have established a connection with someone. If an ISTJ is flirting, they may display signs of loyalty and commitment, such as being reliable, punctual, and following through on their promises. They want to show that they are serious about the potential relationship.

It's important to note that ISTJs may not exhibit all of these signs, as everyone has their unique way of expressing interest. Additionally, ISTJs may take their time to open up and show their romantic intentions, so it's essential to be patient and observant when trying to determine if an ISTJ is flirting.