Conjecture & Appearance: げ vs っぽい vs 気味, に違いない vs に決まっている
The げ / っぽい / 気味 trio is best decided by grammatical behaviour, not by meaning — each attaches differently and has a very different vocabulary range.
The patterns
げ
looking ~ (literary; narrow vocabulary)
彼は自慢げに話した。
He spoke with an air of pride.
っぽい
-ish; tends to be (wide range, not only negative)
彼は子供っぽい性格だ。
He has a childish personality.
気味
a touch of ~ (slightly negative tendency)
最近、疲れ気味だ。
I've been a bit run-down lately.
に違いない
must be; I am certain (most common)
彼は疲れているに違いない。
He must be tired.
に決まっている
of course it's ~ (offhand, subjective)
彼が勝つに決まっている。
Of course he'll win.
一方だ
keeps getting ~ (one-directional trend)
彼の病状は悪化する一方だ。
His condition keeps deteriorating.
つつある
is in the process of ~ (written, gradual)
状況は改善しつつある。
The situation is gradually improving.
Decision procedure
For げ / っぽい / 気味: what comes after the blank?
For certainty expressions: how objective is it?
Commonly confused
げ / っぽい / 気味 — attachment rules
げ behaves like a na-adjective but only accepts a narrow set of fixed emotion words (得意, 自慢, 悲し, 楽し, 寂し, 不安) — you cannot coin new ones. っぽい behaves like an i-adjective, takes a wide range, but never takes だ (疲れっぽいだ is wrong). 気味 is a noun, wide range, but only natural with a negative tilt (得意気味に is unnatural).
一方だ vs 一方で
These look alike but are unrelated. 一方で = "on the other hand" (contrast). 一方だ attaches to a dictionary-form verb and means "keeps going one way" (悪化する一方だ).
に違いない vs わけがない
Equal strength of conviction, opposite direction: わけがない = "no way it could be", に違いない = "it must be".