Mainly through documents. Marriages, births, and residence during past postings generate records in several countries, and dependants may hold different nationalities from the applicant. Eligible spouses and unmarried children under 21 may seek L2, and a child's eligibility depends on age and marital status at the relevant time.
Check each dependant's own route
Nationality affects whether a visa step applies and where it would take place, so a family that moved together abroad may not follow one path now. Collect civil documents and any translations early, and track the ages of children against the timeline. If a spouse expects to work, have that assessed on its own facts, noting that a spouse in valid L2S status is authorised to work incident to status.
Collect the civil documents early, because a family that has moved between countries generates marriage, birth, and residence records in several systems and each may need certification or translation. Nationality affects whether a visa step applies and where it would take place, so a household that travelled together abroad may not follow one path now. Eligible spouses and unmarried children under twenty-one may seek dependent classification, and a child's eligibility depends on age and marital status at the relevant time, which makes tracking birthdays against the case timeline a practical necessity rather than an administrative nicety.
Where a spouse expects to work, have that assessed on its own facts rather than assumed from the principal's classification.