I had prodromal labor for weeks - lasting up to 4+ hours a day, I kept asking the same thing, how will I know when this is real?
Lost my plug on a saturday, nothing more all week so I finally let my midwife check me on thursday, 3cm diliated and 75% effaced - we thought a baby for sure that weekend. Nope, no baby monday, tuesday, wed, so on the next thursday I let her check me again, 4cm and fully effaced - I was a week overdue at this point. So I let her do cervical massage. Contractions all the way home - yippee! Then they stopped!!!!!
Next morning I had one contaction while DH was rubbing my lower back, and went "this is it" - it wasn't more intense than the prodromal labor, or longer or anything, it was just "different". DH wasn't convinced, so I ran a warm tub and got in to soak. 5 min, another one, 5 min, another one - now he was convinced! 4 hours later I was pushing!
It drove me nuts that everyone kept saying "you will just know" but from one momma with severe back issues, who suffered through back pain my whole pregnancy - you will know, it will feel different!