What DallasNChains said would apply to catch a subscription starting or renewing it monthly, ‘twitchnotify’ makes an announcement you can use to trigger an action.
What 3ventic has said is the more conventional message which can give you the current status of a user at any time as long as they said something. So if you have doing an action on a chat trigger !command then you can check the IRCv3 tags:
@badges=global_mod/1,turbo/1;color=#0D4200;display-name=TWITCH_UserNaME;emotes=25:0-4,12-16/1902:6-10;mod=0;room-id=1337;subscriber=0;turbo=1;user-id=1337;user-type=global_mod :twitch_username!twitch_username@twitch_username.tmi.twitch.tv PRIVMSG #channel :Kappa Keepo Kappa
“subscriber=0” not subscribed. “subscriber=1” they are subscribed.
Of course there is the least popular method of doing an API call using an oauth:token generated by the streamer of that channel.