Lighting up an old vacuum tube
Don't fear the datasheet.  Tube datasheets are a fun read.  Google for "12DT8" and the datasheet is the first or second link.

Here are the relevant parts for your question:





The heater connections are labeled "H".  I believe you will find many tubes have the same pinout.  If you look at the radio itself there is usually a label listing the tube complement.

For the heater locations of the unmarked tubes, look to the radio they came from.  The heaters are all wired together, either in parallel (for transformer radios) or series (for "AC/DC" sets).

Anyway, lighting up the tubes is kinda boring, just using them as light bulbs.  Why not get the old radio working again?  There is a lot of old radio service information on the web these days.