Most of the time you can tell, at least then nude. A lot of fake breasts have some giveaway signs like them looking like balloons or being spaced too far apart. Occasionally there are some that are done well enough that it can be hard to tell, but those are a minority. Now if a lady with fake breasts is wearing something that does not really expose cleavage you likely can't tell.
You definitely can tell when they are next to someone who has real ones. At the nudist resort I go to they stand out like a sore thumb for the most part. My wife did have a lift years ago which is not the same as fakes. And they do look good.
I was at a funeral where one of the mourners - a carer for the deceased - had an obvious boob job. She was making the most of her investment with a very low cut top. Yes she did distract most of the males there, encouraging thoughts not quite appropriate for a funeral.
I think a boob job is more obvious when she is naked. When clothed the bra can support the breast. When naked they should hang and move freely, swaying or bouncing as she moves. Enhanced boobs have tense skin - skin that is tight due to the extra filling. Also natural breasts , unless small, will fall to the sides when she lies on her back. Medics rely on this if they have to give heart massage to resuscitate a woman.
I am a great lover of female breasts, be they large or small, but unlike many men my preference is for small breasts.
It is certainly possible to tell the difference between real and artificial breasts however by the way they move when body position changes. I knew a girl who had small but very interesting breasts, but she didn't think they were big enough, and I spent ages trying to talk her out of having implants, all to no avail.
She went ahead and had the op and the result was very good, if all that was required was that they should fill a bra better. We used to visit the same nude venues and the next time I saw her she came over and asked me what I thought. Whilst she was standing they were OK but did not hang properly, as if they were two lumps stuck on her chest. I asked her to lie down and they looked even worse, like two hemispheres sticking straight up. She asked me if I would like to feel them, so I did and they didn't feel right either - it was obvious which was breast and which was plastic!
I asked her what she thought about them and she said ' Mm, they're alright.' Not a resoundingly favourable opinion! She then asked me what I thought about them, and I had to say that they were everything I had expected, and what I had tried to warn her about.
I don't think other friends were very happy about them either, and subsequently she had the implants removed, and was back to her original size.
The next time we met she was looking very happy and let me feel her breasts again, which I had done many times previously, and I was delighted to tell her that they felt as lovely as they always did before her op.