How is it possible for God to see believers in Christ on the cross before they were born?
It is possible because God is the omnipotent, omniscient God; with Him nothing is impossible. When a person believes the Gospel, God saw that believer in the person of Christ on the cross, so their future birth is no barrier to His seeing them in Christ.
Les explains that Paul writes as the believer who has reckoned old Adam crucified and dead, though old Adam remains an influence in Christian life. The moment a person believes the Gospel, God saw that person in the person of Christ on the cross. This requires God to be the omnipotent, omniscient God He is; otherwise it would never be possible, but with Him nothing is impossible. The same context notes that a fetus's blood circulatory system originates with the Father, not the mother, which is why Christ's blood was divine and sinless.