Can I still get a South African passport if I haven't lived there since I was a child?

You are still a South African citizen.

The South African Citizenship Act 1995 sets out the rules for loss of citizenship in Chapter 3, Section 6:

6.(1) Subject to the provisions of sub-section (2), A South African shall cease to be a South African citizen if --

(a) he or she, whilst not being a minor, by some voluntary and formal act other than marriage, acquires the citizenship or nationality of a country other than the Republic (b) ...

You would have lost your citizenship once you had acquired British citizenship, but as you were a minor this section does not apply.

Note that even as an adult it is possible to retain South African citizenship, but one must apply for permission to do so before acquiring a second nationality.

Since your question is, “So how could I check if I still have citizenship?“. My answer is call the embassy. Who ever said that you have to go into the embassy itself.

My South African wife has never set foot in the South African Embassy in Washington D.C.. It is very unlikely that she has ever set foot in the South African Consulate or the Honorary Consul of South Africa in Texas. Yet, she was able to handle all official business through mail and phone. This included renewing of passports. The caveat to this is that dealing with South African bureaucracy takes an overabundance of both time and patience.

However, when it came to expediting a passport renewal, recent changes in the office of home affairs made the wait time for a new passport intolerable. Couple this with the fact that new registration requirements made it necessary for her to obtain a South African Smart Identity Card. Something that is nearly impossible to do if you have never been issued the old green bar-coded identity book. At least with any speed. South Africans who have not lived in South Africa since their 15th birthday are very unlikely to have been isssued the old green bar-coded identity book.

Long story short, it took far less time to travel to South Africa en route to our destination where we would need the South African passport and get a passport and ID card as opposed to six months by mail. It was a matter of spending two weeks (don’t let any website or government official tell you it will be shorter) in South Africa. Luckily, my wife’s expiring passport was still valid for a month or two past our estimated return date.

The lesson learned here was, if you want to retain your South African citizenship, start the process of getting your national ID card and passport a year before you may need it. Otherwise, you may be in for a long plane ride and a que line almost as long.