Since bartenders cannot legally serve inebriated individuals alcohol, they notice if you're wobbling, standing up straight, walking to the bar alone, or leaning on others.
Always carry your drink with you to avoid anyone spiking it. Bartenders try to maintain a watchful eye over these drinks but remember, their jobs get busier and busier as the night rolls along.
Bartenders notice customers' reactions to their drinks: elongating consumption means their tastebuds don't like the concoction, but guzzling a drink means they'll return to the counter.
Boisterous groups who flaunt disrespect and don't tip well have a higher chance of the bouncer removing them than respectable groups who tip well and engage with the bartender without disrupting their workflow.