What are typical signs that someone is searching for a job?
Hard to tell, as there is no "standard" for signs that indicate someone is job-hunting. Some people could show no signs, some may be less focused, some may be more irritated, others could be more friendly/cooperative, etc.. it will depend on the nature and personality of this specific coworker.
There are many possibilities on why your coworker is distracted/leaving earlier. It could be family issues, health issues, or well job-searching...
However, as someone stated in comments, the real issue here is the communication and cooperation between you and this coworker, and that is what you should be focusing on solving (instead of the why is he behaving that way).
I suggest you approach your coworker, privately, and try something like this:
Hello Joe. I have been getting the feeling that you have been worried by something lately, is everything ok? If I can help in something do tell and I'll gladly assist you.
Now, I must also say that I feel that lately our communication and interaction has not been in its best shape, and that has started to affect me in X and Y tasks. What can we do to overcome this situation? What are your thoughts on this?
This will show politeness and empathy towards your coworker, as well as a positive attitude and disposition to find a mutual solution. From here you can work out what you two need from each other to make this interaction as smoothly as possible.
Now, if this does not work, or does not work as fast as you need, the next course of action is to raise this with your boss. Explain to them professionally and objectively the tasks and requirements you have been having issues advancing with, and ask you boss what he/she suggests to do.
Don't mention your suspicious about your coworker job-searching, as that would be speculating and will not help solve the core issue. Again, keep it as objectively and professionally as you can, stating facts and what you need to be productive. Your boss should know how to handle this from that point.