How To Be On Guard?

Well, I use a digital scale and I carefully study any possible purchases, but I don't think common date U.S coins are heavily counterfeited. Not saying you shouldn't be careful though, I'm very paranoid about my coins.

Well, it sounds like you don't want to buy from e-bay. So where will you be buying from?

If your coin dealer sells it to you or you buy it at a coin show, it probably meets the caliper and weight test - after all, coin dealers aren't fools.

So how DO you check?

There are Walking Liberty half dollars out there (and coins from many other series) that have fooled coin collectors for decades. It doesn't have to be uncirculated or worth a lot of money. Counterfeiting has been going on as long as money has been minted and the better counterfeits will pass your "scientific" analysis.

Perhaps one of the most counterfeited coins is the Sacagawea dollar - the coin everyone loves to hate. Go figure.

A dealer I know that has been in the buisness for possibly 50 or more years purchased a 1916D Mercury dime in a 2x2 and knew it was real. So he sent it in for grading and it came back in a body bag. Of course he resent to another TPGS and it came back graded and slabbed.HMMM
Another dealer I know that also has been around for a real long time has a tendancy to massively undergrade his coins. I've been trying to tell him that for years but in the meantime I come up with some really great deals.
One dealer at a coin show was telling me he has been taken in by counterfeits about 25 times so far this year and he too has been around for a long time.
I really don't think because you think you know coins you can really find all the fakes to easily. I'm sure there are many so called experts with a fake or two that don't know it yet.
And as to which coins have been messed with. Note all the reconditioned 1943 Steel Cents. I presently have 2,650 of them. Many have been replated with either Zinc, Tin, Chrome, Copper and for all I know, Radium. I'm trying to accumulate an entire roll of the chromed ones and getting close. Ever wonder why someone would do that to a $0.01 coin that is worth $0.25 and try to sell it for $0.25? One dealer by me sells them for 3/$1 and has a hard time doing that.
And wasn't there this thing not long ago about someone making Nickels? I'm just waiting for the counterfeited 2009 Lincoln Cents to hit the market.

Not to talk about counterfeit per say but I don't understand sellers who don't show both sides of a coin. Does it cost more as a seller to post two picutres ?

Posting one pic is the cheap, lazy, and wrong way to sell a coin. Even if you do not want to pay the meager pittance it costs to add a second pic, you can always use a photo hosting service like Photobucket at no extra charge at all.