When trying to determine if an ISTP (Introverted, Sensing, Thinking, Perceiving) is flirting, it's important to consider their typical behavior and communication style. ISTPs are known for being reserved and private individuals, which can make it challenging to identify their flirting signals. However, there are a few key indicators to look out for:

1. Increased attention: ISTPs are generally observant and detail-oriented, but when they are flirting, they may pay even closer attention to the person they are interested in. They might make an effort to remember small details about you or your conversations, showing that they are genuinely interested in getting to know you better.

2. Playful teasing: ISTPs often have a dry sense of humor and enjoy playful banter. When flirting, they may engage in light teasing or sarcasm as a way to create a connection and show their interest. This teasing is usually lighthearted and not meant to be hurtful.

3. Physical touch: ISTPs are typically not very touchy-feely individuals, so if they initiate physical contact, it can be a strong indication of their interest. This could include casual touches on the arm, shoulder, or back, or even playful nudges or pokes.

4. Active listening: ISTPs are generally good listeners, but when they are flirting, they may exhibit even more attentive listening skills. They might maintain eye contact, nod, and ask follow-up questions to show that they are genuinely interested in what you have to say.

5. Initiating one-on-one time: ISTPs are independent and value their alone time, so if they make an effort to spend time alone with you, it could be a sign of their interest. They might suggest activities or outings that allow for more intimate conversations and personal connections.

6. Sharing personal information: ISTPs are typically private individuals, so if they open up and share personal stories or experiences with you, it can be a sign that they trust you and want to deepen the connection. This can be a significant indicator of their interest in flirting.

7. Increased availability: ISTPs are often focused on their own projects and interests, so if they start making themselves more available to spend time with you or help you out, it can be a sign that they are interested in you. They might offer assistance, support, or simply make themselves more accessible to you.

It's important to note that ISTPs can be subtle in their flirting style, and their reserved nature may make it challenging to interpret their intentions accurately. Therefore, it's always best to communicate openly and directly with them to avoid any misunderstandings.