Do you know about Frank's tubedata.org? Its got the most info on tubes of pretty much any site. Apparently there is a good list of substitutes on that page, but I don't know where.
Basically, how you would tell if you can substitute one tube for another is look up the data on both tubes of interest and compare. You don't have to know what it all means, just compare data... Here is what you should focus on in determining if they are equivalents:
of course the pinouts need to be the same, this is almost always in the data sheet, a drawing of the pinout. the pins to the different parts of the tube should be the same.
Amplification Factor or mu (greek letter)
Plate resistance or Rp
Max Voltage (on plate/anode)
Transconductance, Gm (in umHos or mA/V)
Plate current or Ip
The above should all be ~1% within eachother for the tubes to be true equivalents. If they are no more than 5% of eachother you can likely substitute one for the other but it doesn't mean its optimal in this circuit.
Other data that you might also note is:
Direct Interelectrode Capacitances, Grid to Plate, spec'd in pf
Max heater to cathode voltage, Vhk
e.g.
http://www.nj7p.org/Tube4.php?tube=01aNote that many of the properties listed on this site are for specific operating points, such as the amp factor, gm, etc. That is because those values change for different points. In this case, just try to pick the same points from sheet to sheet, most easily noted by the plate voltage. This site gives a few different plate voltage operating points and data for each.
