Verifying Genuine Coins

Dealers that belong to the PNG (Professional Numismatics Guild) used to have a lifetime guarantee for the coins genuineness. I imagine they still do. Best advice is from rkingston above, buy from a trusted dealer. And read and learn and look at large numbers of coins so you learn what to look for. Stick around the "World Coin Section" on this forum and you can pick up a PHD in Counterfeit Detection, it is a common topic over there.

The problem isn't with the PCGS holder being tampered with, the problem is fake PCGS holders that are coming from China with the fake coin already in them.

Due to the excessive fake coins, fake slabs, reopened and resealed slabs, there is no longer a sure way of knowing what is real and what is not. I've met dealers that were fooled with fakes. I met a dealer that had purposely opened a slab, changed the coin, resealed it just to show people they are not really safe anymore. I suspect it is possible that many members right here have fakes in their collections and don't know that. I'm sure there are dealers that too have fakes for sale and don't know they are fakes.
Some have said it is easy to detect a fake Silver Coin by weight. No longer true. China uses real materials in many of their fakes to make them more realistic since their purpose it to sell as a rare coin, not bullion.
I try to only buy coins at coin shows where there are many professional coin dealers. This is by no means fool proof, it only means for a suspecious coin, there are numerous people to ask if they think it's real or not.
Buying on ebay or the internet is sort of gambling. You spend money and have no way to really know if what your going to get is real or not. No one to verify. No way to really know what you have until to late in many cases.
For me since I'm not selling any of my coins, ever, I might have a fake or many and just don't know. I'm happy with what I have and in the future, whoever gets this mess, will have to worry about fakes.

Sadly, I think Carl is correct. There just isn't any way to be 100% sure every time with every coin. There's good advice above to minimize your risk and beyond that, I think you have to be conscious of your financial exposure and decide if you're willing to accept that. You could employ every precaution and there would still be a set of potential circumstances where you get burned.

That last one won't work for me; I've proven to be a lousy judge of character. I also have bought a counterfeit, a raw $10 Gold coin I got on ebay many years ago. The seller was a Calif coin store with a huge 100% rating, and this was back in '98. I didn't get the coin submitted to TPG until some years later, and it was a blatant counterfeit (MM shows heat marks around it, local dealer was very embarrassed he hadn't noticed that, and he'd been in the business 40 yrs). I don't believe the seller knew it was counterfeit, and I carry it around as a reminder of what can happen.

As already noted many people could not or do not know who is telling the truth and/or lying. And then too, today may good dealers do not know the difference in many of the well made fakes either. Weighing coins, using a magnifyer, asking a freind, etc. are still just taking chances today. The olden day fakes where usually obvious due to little knowledge on how to make them. Today, in China, they are using soficticated machinery to maake our coins and with high grade materials. True Silver may well be up there in value but a coin worth ten times the Silver content to a counterfeiter is well worth that.
It is a shame that as any hobby or anything else grows in popularity, so does the amount of people that exploit that.
I always get a kick out of flea markets products. One of my favorites is batteries. I've seen DinoCell, Dinocel, Durasell, Duracel and many others to the point I know longer know the right spelling.
At auto supply stores I'm usully asked if I want the real replacement part of the so called after market product. I've always wondered about the risk with brake pads made in China.

I would say as well, buy from a dealer who has been in business for years upon years (don't take their word for it and be respectful always). It's more expensive to buy from a reputable dealer sometimes but the high-end stuff should not be played with, for it pays in the end to be safe. These kinds of dealers have spect years building their reputation to where it is presently and they won't jeopardize that for a measly $100-$1000.

Never EVER be intimidated into committing to a coin just because you've been scrutizing it for 10 minutes+ straight. The money/power is in your hands. Turn them in when you're ready.