If your crush is into you, they’ll likely follow you on all of your social media accounts to learn as much about you as possible. Notice if they send you messages on Instagram, tag you in posts on Facebook, or interact with you often on other forms of social media. If your crush only texts you late at night or when they’re bored, they might not be as interested as you are. Someone who really likes you will text or call you whenever they’re thinking about you, such as when they have free time during the day and want to know what you're doing, or when something exciting happens and they want to share it with you. If your crush texts you in the morning, this is a good sign—it means you’re one of the first things they think of when they wake up. Someone who is super interested in you will want to talk to you all the time. If you and your crush have lengthy conversations often, or even just send a few short texts throughout the day, it’s likely that they have strong feelings for you. This could also be a sign that your crush thinks of you as a very good friend. These could be messages over text or over social media. If you send your crush messages and they don’t respond often, they might not be interested. Check to see if your crush always responds to your messages, even if it isn’t an immediate response.  Be aware that your crush might not be able to respond as soon as you send a message. If they respond after a few hours, this is still a sign they like you, while if they don’t respond for several days, they might not be interested. Don't rely on this to determine whether your crush definitely likes you, as responding to messages promptly is also just a very courteous thing to do.

TL;DR in Vietnamese:
Chú ý mức độ tương tác giữa bạn và người ấy trên mạng xã hội. Chú ý thời điểm họ nhắn nhắn tin hoặc gọi điện thoại cho bạn. Để ý số lần họ nhắn tin hoặc gọi cho bạn. Người ấy có thường xuyên trả lời tin nhắn của bạn không?