The disciples recognized the Holy Spirit at Pentecost because they knew Jesus intimately through daily time together. Walking, talking, and sharing meals with him meant they could instantly identify when he fulfilled his promise to send the Spirit, as described in Acts 2.
The disciples walk with God, talk with God, fellowship with God, ate with God, ministered with God, did everything with the manifestation of God in Jesus. So they became familiar with God. They understood what God felt like, what he sounded like, how he operated, how he gave instructions. They were able to relate with God and know that, man, I, I've spent time with this person, I've got intimate with this person, I've built relationship with this person. So if this person then leaves, if he comes back, I will know. So what happened on the day of Pentecost is that the promise was waiting to come. In the moment that the disciples felt that familiar presence, they was just like, this is the promise. Now I, I want us to turn quickly to Acts chapter 2 so that you can understand the context of how the Spirit
