Broken plastic guide on tubes - how to tell which way the tube fits in socket?

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.

Is the entire plastic pin gone?

ozzy, that would work if there were other tubes to check but in this set up, it's the 6SL7 that is broken, along with both my 6SN7>6F8G tube converters that I'd like to not replace.

imhififan, thanks, that's a great replacement piece. Not sure how I would know which direction to install it however.
arion, yes, the pin is completely gone.

I have the same experience, and had to resort to using magnifiers to try and determine if there was a little more of a chip where the directional 'tooth' had snapped off. Fortunately, there was just a tiny extra piece missing where the tooth protruded. Once found, it is a great idea to mark with a black dot on the tube base, just where this tooth pointed.