How would warriors have known friend from foe in the midst of battle? Assuming there were no standard uniforms, how did they know who to kill and who might kill them?

As the other answers have pointed out, this was not usually as much of an issue as you might think.

It seems this caused more issues in the early modern period, and so the usual attempt to deal with the problem in the English Civil Wars was via 'field words' and 'field signs'. So for instance, at Naseby the Royalists wore beanstalks, while their word was 'Queen Mary' and the Parliamentarians' 'God our Strength'. Obviously these were not very strong methods of identification (marks can be removed, the enemy know the word immediately, as it was used as a war cry), and could cause their own issues, as at Cheriton in 1644 when both sides had 'God with us' as their cry and were wearing white scraps in their hats for recognition.

The pass word was used in other situations as well. In Tokelau there's a story that in the old days when the atolls used to fight the fleet of Fakaofo landed on Atafu. In those days supposedly the two sides would fight during the day, but they would rest together, eat together and talk together during gaps in the fighting. The Atafu men persuaded the men from Fakaofo to go bird-catching with them, and then privately arranged that when asked how many birds they had they wouldn't use the normal counting (a single, a double and so on) [I'm working from a translated book here, so I can't tell if birds use a special numbering system or not], but would say 'a single and your kalehi', 'a double and your kalehi' and so on. Then, when most of the party climbed the puka trees, some of the Atafu men remained on the ground with their clubs, and as the hunters climbed back down would ask them what they had caught, and kill the men who answered wrong. The last Fakaofo man encountered a coconut crab on his way down, and discovered that it was covered with blood, so waited til dawn, jumped down and escaped, but the others were all killed.

Very similar stories to this, with one group using apparent friendship to seperate the other's warriors so as to kill them one by one, are very common in Māori history, but the detail of the password is never included, probably because all the ones I'm aware of occurred during the day, when it was easy to tell who was who (it was usually carried out by relatively small groups who would know each other to sight).