Here is what the traditional, orthadox understanding of The Trinity Says:
1. God exists as 3 distinct persons
2. Each person is fully God
3. There is One God.
We believe each of these are taught in scripture:
#1 is true, based on the son praying to the father, the father sending the son, the son sending the spirit, the father creating the world through the son, the son being "with God" from the beginning.
#2 is true, based on many passages ascribing diety to Jesus, and a few select passages equating the H.S. with God.
#3 is true based on the entire Old and New Testament teaching about God and idolatry.
Beyond this, there is room for disgreement about how this actually works.
But, Tri-theists reject #3, Mormons reject #2, and Modalists reject #1. For them to do this, they must ignore or explain away the many passages where the persons of the trinity relate & talk to each other, and "send" each other places.
If modalism is true, there is also the issue of Jesus taking the father's wrath for our sin, and crying out, my God, my God, why have you forsaken me.
Where I'm at right now is this: I believe there can be oneness/modalists who simply have not looked carefully at the scriptures, and misunderstand how the trinity works, and they are true Christians...
But...It seems that to maintain the modalist position in the face of the scriptures that show the son's relationship to the father, one has to disregard a multitude of scriptures, and one cannot also explain how Jesus took the punishment for our sins...and such a continued rejection of scripture would give us good reason to wonder if they are truly a believer who is willing to let scripture shape his/her beliefs.
That said, still, I would question them more about their gospel beliefs than their trinity beliefs to find out if they were a christian.