How was it decided who would be on the front lines in the revolutionary war and other older wars?

In the case of some armies, like the Brunswick armies? How good your moustache is. More can always be said on the matter if anyone would like to expand further on the little quirks of Early Modern warfare; for the meantime, these two posts by u/dandan_noodles should go some way to addressing your concerns:

• On your thought of it being a death sentence, one is reminded of similar considerations before gunpowder, and is minded that the nature of gunpowder weaponry may well equalise the risk;
• and in conjunction with u/PartyMoses, also examines placement, motivation, and the relevant risks.