7 months after double eyelid surgery, Rubbed the corner of my eyes in my sleep, what are the signs of a loose suture?

Sutures loosen up over time. It doesn't necessarily indicate a complete break, it may simply be that the tissues surrounding the sutures are loosening up, which is normal. If you have a complete break of one of the sutures, then the crease may look truncated or change shape. If there's a suture scratching your eyeball, then it's an urgent issue, otherwise, you can go see surgeon who performs this type of surgery on a regular basis.

In some circumstances, the crease will disappear.  In other cases, the scaring associated with the suture is sufficient to maintain the crease.  There are many deficiencies of suture technique, so it is common to eventually have an open incision surgery to make the eyelid crease permanent. If your crease has not changed and there is no exposure of the suture material, you are likely fine.  If you are concerned, I recommend seeing an oculoplastic surgeon.

Sutures do loosen over time, however, at 7 months you should be completely healed and not depended on the sutures.  If your worried then please go see your surgeon.