The post mode was an option on the old journal entry and comment forms, but was removed for the new versions.
From a usability standpoint, having the post mode on the form would be proper. Most users use one posting mode, but occasionally need a different one. For instance, I use "Plain Old Text" almost exclusively, but there are times when I want to use something else, such as "Code" when I post source. Rather than forcing a user to change preferences, it would be better to just allow the option - even if it is used infrequently. This is one instance in which a 90/10 decision detracted unnecessarily from usability.
The post mode is now figured out by looking at the contents of your post.
If you have a post where it guessed wrong, let me know and I can take a look. But basically, IIRC, if you include a P or BR, it will do HTML Formatted, otherwise it will do Plain Old Text. And if it starts with PRE then it puts the comment into Code mode.