Q: Can I tell at a glance if my preamp is tube or transistor?
The answer is yes, you can tell instantly.
There is a perception in the pro audio manufacturing industry that solid state preamplifiers (transistor, FET, integrated circuit) are ordinary and vacuum tube preamplifiers are special.
(Or perhaps that's a perception put about by clever marketing.)
So a tube amplifier will always be very clearly identified as such, so that a potential purchaser can see that it is supposed to be special.
So you will see 'TUBE' in big large letters on the front panel.
Or you will actually be able to see the tube(s) through a window in the case. Hi-fi manufacturers sometimes mount the tubes externally, but that would be too fragile for studio use.
If you can't see 'TUBE' written on the front, or a window displaying the tubes inside, then it is almost certainly a solid-state device.
If you come across an exception, please let us know!