‘And it came about that, while Apollos was at Corinth, Paul having passed through the upper country came to Ephesus, and found certain disciples, and he said to them, “Did you receive the Holy Spirit when you believed?”
When Paul arrived in Ephesus he ‘found' certain disciples. Perceiving the lack of any signs of the Holy Spirit in these men, in spite of what they appeared to believe, Paul asked them, “Did you receive the Holy Spirit when you believed?” Without the Holy Spirit they were not members of the body of Christ, nor were they full Christians.
‘Found certain disciples.' They would probably be Jews whose faith had been extended by acceptance of the teaching of John the Baptiser and belief in the Coming One. They were ‘almost-Christians'. They followed ‘the way of righteousness' and have therefore to a certain extent entered under the Kingly Rule of God (Matthew 21:31-32). Indeed a number of the Apostles had once been such disciples of John. They can be classed as ‘disciples' because in their own way they are believers in Jesus and desire to follow Him (although we should note that Luke does not necessarily always refer to true believers when he speaks of ‘disciples' - Luke 6:17; Luke 19:37; Luke 19:39). We are not told how he found them. Note that they are honoured by being given precedence to his visit to the synagogue. They are dealt with first and are seen as a unique and precious harvest-field to be garnered. Perhaps he had learned of them from a previous visit and had now sought them out.