(1) How will people know it's him - will it be through what he does? How will anybody recognise him because nobody alive would have known him?
Matthew 24:29-31: "Immediately after the distress of those days "the sun will be darkened, and the moon will not give its light; the stars will fall from the sky, and the heavenly bodies will be shaken.' (Isaiah 13:10; 34:4) At that time the sign of the Son of Man will appear in the sky, and all the nations of the earth will mourn. They will see the Son of Man coming on the clouds of the sky, with power and great glory. And he will send his angels with a loud trumpet call, and they will gather his elect from the four winds, from one end of the heavens to the other." (Also Mark 13:26)
Matthew 26:64: "Yes, it is as you say," Jesus replied. "But I say to all of you: In the future you will see the Son of Man sitting at the right hand of the Mighty One and coming on the clouds of heaven."
Luke 21:25-27: "There will be signs in the sun, moon and stars. On the earth, nations will be in anguish and perplexity at the roaring and tossing of the sea. Men will faint from terror, apprehensive of what is coming on the world, for the heavenly bodies will be shaken. At that time they will see the Son of Man coming in a cloud with power and great glory."
1 Thessalonians 4:16-17: "For the Lord himself will come down from heaven, with a loud command, with the voice of the archangel and with the trumpet call of God and the dead in Christ will rise first. After that, we who are still alive and are left will be caught up together with them in the clouds to meet the Lord in the air."
Revelation 1:7-8: "Look, he is coming with the clouds, and every eye will see him, even those who pierced him; and all the peoples of the earth will mourn because of him. So shall it be! Amen. I am the Alpha and the Omega, says the Lord God, who is, and who was, and who is to come, the Almighty."
I don't think anybody will be in any doubt as to who it is that comes, because of the earth-shattering and cataclysmic events that will accompany his arrival.
(2) Will there be disagreements between the various Christian denominations as to whether it's really Jesus or not?
Yes, because the anti-Christ will deceive many, and not just Christians, as Fiona M points out.
One answer quoted from a Watchtower magazine to give the Jehovah's Witness point of view about the second coming but fell short of saying they believe this second coming has already taken place in 1914 and that it was invisible. Only they 'saw with eyes of understanding' (and with hindsight) what nobody else did - that Christ Jesus took up his heavenly throne and has already started to reign. Christ Jesus, they say, will not be returning to earth. To my knowledge, this viewpoint is not shared by any Christian denomination.
Whatever opinions people hold on this subject, the Bible makes it very clear that when Christ Jesus returns, with the angels, every knee will bow and nobody will be in any doubt as to who he is and why he has returned.
Personally, I think it is futile to speculate on 'end-time matters'. All that does is to cause divisions and to take people's minds away from the good news of the gospel message which is all about pointing people to Christ Jesus as Lord and Saviour. There is little to be gained in spending time wondering about the when and the how (because no man knows the day or the hour). Rather, Christians should be living each day as if the Lord was going to return tonight - walking in faith and in the assurance that God's timing and God's justice are perfect.