"So my question is, in the perspective of a guy, How do I know if the girl is showing interest or not without actually going out?"

The answer is, unfortunately, you can't know, whether from the perspective of the guy or girl.

The reason is: the medium of communication is texts, or even voice/video calls, of ascending effectiveness. Still, video call is inferior to in-person getting together for a drink/meal.

The reason is when there's a physical person to person communication, there's the body language, facial expressions for interpretation. 95% of communication is visual. Believe it or not too, chemical exchange also takes place in terms of smell. Google "pheromones" to find out.

Texting is the poorest form of communication. If you had texted for a long time, you'd know misunderstanding often takes place too due to language use.

It works both ways: a person you text with, may appear unattractive to you, but on meeting up may prove otherwise.
Similarly the opposite can take place.

So the best bet is to go out at soon as one thinks the preliminaries have been completed.

Good luck.