When trying to determine if an ISTJ (Introverted, Sensing, Thinking, Judging) likes someone, it's important to consider their typical behavior and communication style. ISTJs tend to be reserved and private individuals who may not openly express their feelings. However, there are several signs that can indicate their interest in someone:

1. Consistency and reliability: ISTJs are known for their strong sense of duty and commitment. If an ISTJ consistently shows up and follows through on their promises, it can be a sign that they value the person and want to invest in the relationship.

2. Active listening: ISTJs are attentive listeners and tend to focus on details. If they show genuine interest in what someone is saying, remember specific details, and ask follow-up questions, it suggests that they are interested in getting to know the person better.

3. Practical gestures of care: ISTJs often express their affection through practical actions rather than grand gestures. They may offer to help with tasks, provide practical advice, or show concern for the well-being of the person they like.

4. Sharing personal information: ISTJs are generally private individuals, but if they start opening up and sharing personal details about their life, experiences, or thoughts, it can indicate that they trust and feel comfortable with the person.

5. Respect for boundaries: ISTJs highly value personal boundaries and expect others to respect theirs as well. If an ISTJ is mindful of someone's boundaries, it shows that they care about the person's comfort and want to maintain a healthy relationship.

6. Increased time spent together: ISTJs are typically introverted and enjoy their alone time. However, if they willingly spend more time with someone, initiate plans, or show a willingness to engage in activities together, it can be a sign of their interest.

7. Thoughtful gifts or acts of service: ISTJs often express their affection through practical means. They may surprise the person they like with thoughtful gifts or perform acts of service to make their life easier or more enjoyable.

8. Nervousness or shyness: While ISTJs may not openly express their feelings, they can become nervous or shy around someone they like. They may struggle with finding the right words or appear slightly awkward in their presence.

9. Loyalty and commitment: ISTJs are known for their loyalty and commitment to their relationships. If they prioritize someone's needs, stand up for them, or consistently show support, it indicates that they genuinely care about the person.

It's important to note that everyone is unique, and these signs may vary from person to person. Additionally, ISTJs may take time to develop feelings and may not rush into a relationship. Patience and understanding are key when trying to determine an ISTJ's feelings towards someone.