Almost 37 wks. with twins, 2 cm dilated 4 days ago. having very bad backache for 2 days. when or how will i know if it is true labour?

Go to L;D: No harm in going to labor ; delivery to get checked out and to see if these pains are causing dilation. And, by the way, congratulations on making it to 37 weeks, you should feel so proud. That's outstanding (and hard, i know, on the back). A check in l;d will not only check your cervix but also the babies' heart beats, which is always a good idea so close to term. Good luck!