You asked two different questions between the title of your thread and the content of your first message.   The answer to the question in the title of your thread; "How to check if plants are in flowering?" is simple. If you are growing outdoors once the summer solstice has come and passed your plants are then in flower. If you are growing indoors once you change your light cycle to a 12 hour long or longer period of darkness your plants are then in flower. Once the light cycle changes to nights that are 12 hour long or longer that triggers a hormone in plants that begins flowering.     The answer to the question in your first message; "How to check if plants flowering???" is equally simple to answer. LOOK AT THEM.