A major concern in bar loss is consistency and accuracy of pours. A bottle/keg should have X amount of pours in it, plus or minus a few. If this number is not matching the amount of drinks rang in there is an issue.
For beer pours, the key is avoiding excessive head. Certain styles(like stouts) are to be served with some head. Wine pours are usually free but these can be measured/carafed as needed. For liquor, some bars mandate all pours are measured. Some bars measure in a jigger only for expensive liquor. Some bars free pour virtually everything. Classy cocktail bars may measure to get precise combinations and flavors.
A trained bartender can free pour with great accuracy. Many are tested to pour out measured(like 1 or 2 oz) into a glass which is poured back into a jigger, to see how accurate they are. Another common test is to line up shot glasses and have the bartender pour a bunch in a row and check consistency. Most pour by count, the rule being .25 oz being one count. For a one ounce drink, the bartender counts to four before stopping the pour. Using this knowledge, you can also count their pour(and the shop might require this)
You will probably be asked to watch for any complimentary drinks given or taken by the staff. Bars have varying polices on this; at some places high spenders and regulars can be comped drinks now and then. Like any service industry, managers frequently can and do comp food and drink for unhappy customers and incorrect orders. At nicer cocktail bars customers can usually send back a drink they truly do not like, and at beer halls with large selections sampling beers is normal practice. Some bars allow bartenders to drink on the job if the customer buys the round as a compliment to the house, others may even allow for more, but this should be noted in the shop.