Usually you can see where the guide was. If you can then place a mark on the tube base or grass with a marker that corresponds with the ex-guide. Accurate location of the mark is important. Then place a mark on the amp that corresponds with the tube socket slot. That should make it easier to align the tube with the amp socket.
Broken plastic guide on tubes - how to tell which way the tube fits in socket?
I have a few tubes (and also a set of 6SN7>6F8G tube converters) where the plastic "guide" with the notch that tells you how the tube fits in the socket is broken off. Any way to tell which way the tube or adapter is supposed to fit in the socket without experimenting and frying the tube?