The prophets of old had tests they had to live up to: mainly the fact that their prophesies had to come true. God more than anyone understands the power of false prophets, so God would have His prophets prophesy to His people, then cause the prophesies to come true. This is why so many old testament prophesies have dual answers, a short term answer that validated the prophet as being a true prophet of God, and a long term answer as well. If a prophet back then prophesied about something, and it didn't come true, they were to be put to death. So, a good portion of a prophet's prophesies had to come true within a short time-frame in order to validate his veracity as a true prophet of God.
The main difference between then and now though is that the prophets operated right there amongst the people. Elijah very well might have asked you to cart some water from the sea to the alter on Carmel. You could walk the street and bump into Isaiah. Populations were small, and folks knew each. When these prophets made their claims, if you hadn't heard it personally, probably your wife, husband, child, parent or sibling had. So, when the prophet prophesied, either you or a close, trusted friend or family member could personally testify to the short term answer first hand.
Now we have vast populations which make validating claims personally very hard, and mass media which can be manipulated at will. We have technology such as head-phones that workers can feed information gathered about people right into the "prophet's" ear. A person can manipulate a "live" televised interview, by simply filming something after the fact, and altering the date on it. I mean really, who has taped episodes of every TBN "prophet" from 10 years ago, independently validating any claims made in 1990?
On the thread about the Taylor guy who claims to have prophesied about 9/11, I made a litmus test that probably sounded rather cynical. I don't believe it was. I think that anyone claiming to prophesy for God in this day and age, should be held to such tests as these in order to validate their claims. This is what every skeptic about the paranormal or supernatual looks for, a validation of the claims, that can be independently verified.
Regarding Acts 2:17-18, which modern day prophets get so much milage (and money) out of: Re-read the context of those words. Those words were not talking about today, they were talking about the day of Pentecost. Acts 2:1-13 is a fulfillment of a prophesy made by Joel in Joel 2:28-32 and repeated by Peter in Acts 2:17-21.
Read especially what Peter says in Acts 2:15-16: "For these men are not drunk, as you suppose, for it is only the third hour of the day; but this is what was spoken of through the prophet Joel."
Since we now have the Scriptures, we can ususally validate most of the claims made by the prophets. The problem is, people don't really study the Scriptures themselves. They let the 'prophets' tell them what the Scriptures mean, and thus lead many astray, and lead much of their money astray as well. Oh, and you can never take a stand against them, because then they will shout out, "Touch not the Lord's annointed!" yet another verse that is taken out of context and used for their benefit.
Sorry to sound so cynical folks. However, I believe that if God does indeed raise up a prophet today, this prophet will be able to superceed all scrutiny by believers and skeptics alike and will be able to have his or her claims indepently verified.
