You mention you don't want Alex to think you agree, but that you do not know where Alex stands, either. You also mention that you don't want to bring it up with Alex after lunch, that you only see Alex at lunch and no other time. This implies you are not particularly close and this is a work-based companionship. In that case, I think you could approach it one of two ways to avoid alienating Alex unintentionally:
1) Ask Alex for thoughts the next time Bob is talking.
If you choose this option, you are doing so merely to understand where Alex is on the topic. Therefore, with a calm demeanor and a neutral tone say something generic like "What do you think?" Mentally prepare for an answer you may not want. If Alex says "yeah, totally agree with Bob" then I suggest changing the subject if you don't want to damage the situation with Alex.
2) Make a generalized statement like "I really prefer not talking about politics at work"
I personally find that completely avoiding conversations about politics at work is best, regardless of agreeing or disagreeing. It will undoubtedly make someone in the proximity uncomfortable (Like you seem to be when Bob is talking). That is also a reason why simply trying to infer to Alex that you disagree with Bob may not be the best action, either. Without more knowledge about Alex's views, you run the risk of alienating Alex if there is an agreement with Bob.
So, the first option would get you a better understanding of where Alex stands first, and the second option would make it clear that you and Alex should continue your lunches devoid of political discussion, and perhaps engaging in conversation that helps you tune Bob out if that is possible.