From what we understand in the scriptures, there are a few things needed for a Stream Enterer. We have to have complete confidence in the Buddha, Dhamma, and Sangha, that this is the way to purification. Through understanding on a deeper level, we have supreme confidence that this is the path leading to the ending of Dukkha. This is one factor when we become a Stream Enterer.
Other factors include gaining insight into the changing nature of the "I", the self, and not being attached to rites and rituals as a means to purification, thinking that if we just perform rituals, it will purify us.
So these three things are necessary to become a Stream Enterer. Also, according to the scriptures, it is impossible for a Stream Enterer to break the Five Precepts. So if someone tells you that they're enlightened and you see them breaking the Five Precepts, then you can conclude that although they may claim to be "enlightened", they are not enlightened in the Buddhist sense. Often times, these people may have had concentration experiences, but if they're unable to keep Virtue, then they're not following the Buddha's path. According to the Noble Eightfold Path, Virtue (Morality) is part of the path leading to the end of Dukkha.
Is it possible for a layperson? According to the scriptures, yes. There are many instances of laypeople becoming Stream Enterers, they actually stayed in the home life, but their Virtue was very good, they kept the Five Precepts perfectly. They had supreme confidence in the Buddha Dhamma, and they understood that it is Wisdom that purifies the mind and enables them to understand the changing nature of the self.
In order to become a Stream Enterer, we have to strengthen our experiential confidence. We have to see into the changing nature of who and what we are. We try to give you types of meditations that help you to forgive, to develop Compassion and Lovingkindness, letting go of "self" on the emotional level. With the Wisdom sides of the practice, we try to give you techniques that help you to see into the Three Characteristics: Impermanence, Dukkha and Anatta (the impersonal nature of things). As we develop insight on these levels, as far as I understand, we're getting closer to that Stream Enterer.
I was once describing my practice to one of my teachers, and he said, "You're practicing too high." "What do you mean I'm practicing too 'high'?!" I said. Steve was with me at the time, and the teacher said, "You're married, you can become enlightened while staying married. Enlightenment on this level depends on purification of your view, not on ending all sense desire!" Then this teacher got up, strode to the alter and pointed at the candles. "Look at these candles: here's a new one, here's one half burned, and here's one that's completely gone. Contemplate the origination and dissolution of the candle, and then apply it to your own body". He then pointed to the beautiful flowers that the devotees had set on the alter, and then he pointed to some that were wilted, and some that were completely dead, and he said, "Beautiful flowers aren't they? Look at what they become. Contemplate the origination and dissolution of the flowers and apply it to your own body." So he encouraged us to contemplate the origination and dissolution of things on the level of the body.
The last part of the question "Are there, at the moment, enlightened Buddhas in the world?
According to what I understand from the scriptures, it's impossible for a person who is not enlightened to recognize an enlightened person's mind. However we do have guidelines. The first one for the Stream Enterer is to see whether they keep to the Five Precepts - a level of Virtue.
See whether they have a great attachment to their self-image. If they do, it would seem that they're not yet a Stream Enterer. If they have a lot of attachment to rites and rituals and believe that performing these will purify the mind, then I would not think they're a Stream Enterer.
A Once Returner (the next level) is said to have all these qualities, and some other ones as well. The hindrance of doubt doesn't arise any more, and sense desire and aversion are very refined, they're no longer strong.
In the Non-Returner (one more level up), sense desire and aversion have been completely eliminated. However, according to the scriptures, the Non-Returner has some conceit, with restlessness and worry coming from that, so that hasn't been destroyed yet.
In the Arahant, or fully enlightened one, all this has disappeared. Fully enlightened, the Five Hindrances will never arise again. Greed, hatred and ignorance have been eliminated.
How are we going to figure that out? How are we to figure out if they have a higher level of enlightenment? We have to go and be with them "for a long time, not a short time." Watch them "with close attention, not without attention," and we have to have enough Wisdom to know how to discriminate, especially in the Wisdom area, because, "it is by conversation that another's Wisdom is understood."