Our church is similar to many here in that we would require immersion before membership, but gladly extend the Table to visiting Presbyterians. The Table is fenced by the statement that if one is born again, and a member in good standing at another Bible-believing church, they are welcome to partake. It is laid on the conscience, then, of the paedobaptized person to examine himself whether he be in the faith. If he lies, he does not lie to men, but to the Holy Spirit. Externally we can accept that his church, which watches over his soul, regards him a member there.
There is no inconsistency in allowing visitors to partake with less examination than it takes to have members--the elders have a charge to watch for the souls of members, and to fence the membership by knowing the members and seeing their testimony--but they have less responsibility with visitors who are under care elsewhere, and perhaps only passing through that day never to be seen again. I have partaken, while travelling, of the Lord's Supper at many paedobaptistic churches (OPC, RCUS, CRC, etc) who accepted my membership elsewhere, and I was most glad to commune with them, so it would be churlish in the extreme to have them barred from the communion at my church when they were passing through.