The short answer is that a plastic surgeon can usually tell if implants are present, but your intimate partner is usually significantly absorbed in almost everything else and doesn't care!
If you have large implants and not very much of your own tissue, implants are easier to palpate.
If someone is "looking" for implants and feels along the areas of your chest/breast area where your own tissues are thinnest (like the sides or bottom below your breasts), implants can be felt.
If you have saline implants that are underfilled, or above the muscle, there is a much higher chance that rippling can be felt or even seen (watch HBO or Showtime late at night for examples).
If you have capsular contracture, your breasts can be unnaturally firm and distorted, even without touching them.
If you have had surgery, you must have scars, but they can be hidden and nearly imperceptible in many cases. Plastic surgeons can find them, and centerfolds have them airbrushed or Photoshopped out!
If you are trying to "pass off" your post-augmentation appearance as "totally natural" to a new partner, I'd advise not making it a big deal at all. (A loving partner doesn't care, and the improvement in your confidence and self-esteem is a much better aphrodisiac than being self-conscious about small breasts).
If you're asking before possible breast augmentation surgery to help you decide, I can tell you from over 2 decades of experience with thousands of patients that 99% or more of intimate partners don't care, and are quite happy their female companion is happy about her breasts, body image, and sexuality. And if a potential partner is "hung-up" on the fact that you have implants, time to ditch that person!
So the best advice is to carefully choose your breast augmentation surgeon so you get the softest, most natural result--make sure you are seeing a "real" plastic surgeon--one who is certified by the ABPS (American Board of Plastic Surgery), NOT some "bogus" board that is not an ABMS (American Board of Medical Specialties) member.
The "American Board of Cosmetic Surgery" is NOT an ABMS member board--the American Board of Plastic Surgery is the ONLY ABMS board that certifies doctors who have 6-7 years of comprehensive plastic surgical training AFTER medical school. All others may end up being an ER doc, family practitioner, or Dermatologist who has decided to perform cosmetic operations without complete training in plastic surgery and certification by the ABPS.
Seek several consultations with experienced ABPS-certified plastic surgeons who do at least 100 breast augmentations per year, and ask to see lots of before and after photographs. Ask who did the surgery in the photos. Best wishes!