Had veneers for 15 years. How do I tell if they are about to fall?

Unlike crowns, usually veneers do not get loose over time.  Veneers usually just "come off" when biting on something very hard or while grinding (bruxing) their teeth.You can certainly have the one veneer replaced, and in the interim your cosmetic dentist can place a temporary veneer while the lab is fabricating your porcelain veneer.However, a lot has improved in the past fifteen years with cosmetic dentistry and porcelain veneering.  New bonding techniques, changes in tooth preparation, better, stronger and longer lasting porcelains (like EMax), just to mention a few things.If you are COMPLETELY happy with your fifteen year old veneers, then just replace the one veneer.  If not, you might consider replacing them all at once (now), rather than wait for one to break every few years.  You should certainly have a better result now than before for many reasons.Sometimes for very short and small teeth, a bonded porcelain crown rather than a veneer will have better retention and be stronger than a veneer.  You dentist should be able to discuss the pros and cons of this approach with you, and then you can make the call on how you want that tooth to be treated.BTW, there is no "average" length of how long porcelain veneers will last that you should go by.  Who is average anyway?  Differences in people's teeth, habits, oral hygiene, plus differences in the dentist's technique and materials use, and differences in the types of porcelain and work of the ceramist.  Too many things involved here to "guess at".  I've been doing porcelain veneers since they were first introduced to dentistry back in 1985.  Not too long ago a patient returned after having me make her veneers 19 year earlier and not one of them have broken or fallen off.  Funny thing was, she returned because originally I had done six for her, and now she wanted ten new veneers that were several shades whiter.  Since using the "old" technique, bonding agents and materials that had easily lasted 19 years, I am very optimistic that by using the newer techniques, materials and porcelain that her new veneers will also last her decades.

It sounds like your
veneers have reached the point of an average lifetime for this type of
restoration. It’s very easy to replace one veneer, but you’ll want to be sure
there is enough healthy enamel to bond the veneer to. For instance, tooth decay
can creep up under a veneer and destroy tooth enamel. Most likely your veneer
just came off because it lasted as long as it could, and you’ll have other ones
follow in the future. You may want to have that veneer cemented back into place
so that all of your veneers can be replaced at once as soon as you’re ready.

This is a great proactive question. Porcelain veneers like most things have a lifespan. The average lifespan of veneers is 10-12 years. This is an average. So it includes patients who have excellent hygiene as well as those who are neglectful. Many patients who take excellent care of their veneers can have them last 15-30 years. You should have the veneers evaluated to ensure they are still healthy, aesthetic, and decay free. If you need to have them replaced it is a pretty seamless process. The veneers can be removed, temporary veneers placed the same day, and new ones can be fabricated in about 2 weeks. The shape, contour, and color may be enhanced with the new veneers, or the new ones can be designed to be nearly identical to the current veneers depending on patient goals and preference.