We realise that visitors from Scotland are not used to tipping in pubs. If you are not sure what to do, just leave at least a $1 bill per drink on the bar after you get your change. The bartender will know what you are doing. If you are sitting at the bar, you can let this pile of dollar bills sit in front of you, the bartender will usually not pick it up until after you have left.
If you are nice to the bartenders, they may even give you a free drink every now and then (although this is always at their discretion - you should not ask for it).