Question

how to know is the substance is gas liquid or solid at room temp?

Answer 1

use a data booklet, or google, if you're not in an exam situation.
Most courses should have a data booklet which gives the temperature at which substances melt / boil

Answer 2

you would know by looking at the melting/ boiling points
if the melting point is below 25 degrees then it would be soild at room temp
if the melting point and the boiling point is between 25 degrees then it would be liquid at room temp
if the boiling point is above 25 degrees then it would be gas at room temp
hope it helps you :)