How can you tell whether a word is use as an adjective as an adverb?

By observing the form/structure/construction of that Word.

for example:

tease - verb

teasing- adj

teasing - n

teasingly -adv

The verbs usually are very simple. They can be known easily. They are not noticeably or specially built.

But adj - are usually formed with the suffixes '-ive', '-able', '-ed' '-ing' or sometimes simply built. (fat, thin, big, tall, sage)

Nouns are also obvious for being built with the suffixes, '-ness, -ment, -tion,...)

But the naming Words should be studied hard. (e.g. remote, bank, Apple, boy,...)

Adv are also obvious. Normally, they are built with '-ly' but some irregular adv like: hard, plain, slow, quick, ...