How does one know that he's reached a Jhana level in Metta meditation? When someone practices Metta meditation towards a beloved person using a phrase,


  e.g. May xx be be free from enmity and danger


what is the counterpart sign (nimitta) in this method and how does the practitioner identify whether he's reached a Jhana level?
IMO, you are mixing apples and oranges. Metta meditation is practiced towards "the state of brahmas" (AN 4.190 and AN 3.63), not one of the Four Jhanas. The "state of brahmas" (brahmapatta) is a state of non-dualistic unity with the world. The way to attain it, is through progressively expanding the context of Metta meditation, starting from yourself, to include a closest person (e.g. mom), then friends, then neutral people, then enemies, then all the world. The sign of having reached the state of Brahma are the thoughts like "I am God", "I am all this infinite Universe", "I am True Goal of all religions and the Real Meaning of Love" etc. It is a very powerful experience.

While Four Jhanas are much more localized around oneself, esp. one's emotional state. Like you say in your question, The First Jhana can indeed be practiced with a help of a phrase, that helps one focus on the topic of meditation (in case of the first Jhana the topic is one's actual state of successfully having separated oneself from unwholesome dharmas). However, from Second Jhana onward, the target state of rapture/bliss/equanimity must be generated directly, with no help of discursive thinking.

To emphasize one important point: Four Jhanas are "mastered", not "reached". They are not states/levels you automatically find yourself in. In other words, you don't practice something towards Jhana level X and then "reach Jhana level X". You practice Xth Jhana itself. After having mastered First Jhana, you then practice Second Jhana and so on. The sign of having mastered the First Jhana is ability to successfully do the First Jhana at will at any time, that's it.