What is the most efficient way of deducing the culture of a lab during graduate recruitment days?
I second @ff524's comment about the "key questions". Not all of them are equally applicable to each person, so decide what fits your situation the best. 

Plan ahead, and try to memorize a few questions you want to ask. 

Here are a few tips:


Talk to the grads in that research group.
Talk to the faculty leading that research group. 
Talk to the postdocs in that research group.
Talk to other faculty in that department (but in other research groups).
Try to observe interactions between grads and their advisors, if possible. 


What you want to elucidate is the following:


What are the expectations in terms of balancing fun/health/classes/research? Does your potential advisor appreciate friends/family, or is he/she more work-oriented? 
What's the expectation in terms of hours/week worked? Does that seem reasonable compared to other research groups? 
Do people in the group seem to get along with each other? (If not, that's a sign that there could be drama.) Do you think you could get along with the people there? 
Are people -- especially grads -- competing with each other for funding? This can put a strain on interpersonal relationships, unfortunately. 
Are people -- especially the faculty and/or senior grads or postdocs -- looking out for their own interests or for those of others (particularly yours)? In other words, will they be on your side and look out for you? 
Are people only interested in their own work, or do they show an interest in your abilities? Do they show an interest in you as a person -- beyond your academic capabilities? 
Does your potential advisor seem to appreciate his/her students? 


If you're feeling really gutsy, you could ask a few more direct questions, but be careful.