You can make a GET request to /admin/customize/user_fields and it will return a list of user fields:
{
"user_fields": [
{
"id": 2,
"name": "Favorite OS",
"description": "What is your favorite Operating System?",
"field_type": "text",
"editable": true,
"required": false,
"show_on_profile": true,
"show_on_user_card": true,
"searchable": false,
"position": 1
}
]
}