Is my alternator kaput?

[Zsolt]

I went to start my car this morning, and it hardly turned over. When we
boosted it, it only ran while the booster cable was connected, but as
soon as it was removed the engine died.

Seems like it is the alternator to me, but I thought I'd ask if there
are any second opinions. Is there anything else that could cause
something like this all of a sudden? I thought it's a stuck solenoid.

(Currently of all my vehicles only one of my mountain bike is running)

TIA,
   Zsolt
   88 5ktq (broken alternator)
   90 V8q (doesn't rev over 2k)