Just flushed the cooling system, removed the coolant hoses and bottom rad from my NC30.

I've run some water through the rad but what should I be checking for to make sure it's still good to use as I haven't ran the bike since I've had it. Some of the fins have been struck from stuff thrown up by the front tyre but apart from that it looks decent-ish for 21 years old.

As long as it holds water without leaking it's ok?

As long as the water flows through it at reasonable speed and there are no leaks it should be fine. The debris and rust tends to collect in the lower part of the system due to gravity.