The Bible gives us two test for prophets: does what they predict come to pass, and do they tell people to worship and serve the one true God.
Even modern-day prophets have had to admit that their prophetic predictions are often wrong, and have tried to explain their failures away or come up with excuses, which should be very revealing of their own status as false prophet.
While modern-day prophets may claim to point people to God, looking at their teachings can give us a very different picture. They do not get their main teachings from the Bible, but from their own revelations and experiences.
Are their any particular prophets you're concerned about?