How to tell if it’s a contraction?

I’ve heard the whole “if your stomach is hard like your forehead it’s a contraction” but my entire abdomen is hard after a certain time at night. And it’s been doing that for two weeks now? No pain or anything else, my stomach just feels stretched and hard at night. Is that normal? Is it the baby’s position? How do I tell it it’s a contraction if it’s always hard like that?