Some tricks to check whether an expression is formal or informal (apart from looking it up in a few kinds of dictionaries or asking your teacher):
- long Latin words (apprehension) and weird Germanic words (strength) are almost always formal (or neutral, so they are OK in a formal setting)
- Use http://corpus.byu.edu/bnc/. Enter your expression in the search box. A table with your expression should appear. Click your expression to show examples of its use. In a column to the left of the examples you shall see where the examples are from. If it’s “conv” (conversations), your expression is informal. If it’s “ac” (academic), it’s formal.
- this blog is not written in formal English