I'm making a timeout command for my own bot since it has more potential than the Built-In one, and I wanted to make a checker if a user is already timed out or not, but as far as I know, there is no variable for that in guildMember! Is there any way I can do it?
Yes, you can use the communicationDisabledUntil or the communicationDisabledUntilTimestamp properties.
The first one returns a Date object with the time when the member's timeout will be removed and the second one returns a timestamp.