How can you tell 1795 us silver dollar is real?

There is no clear answer to that. You have to realize that the 1795 Silver Dollar is often times reproduced. The first thing to look for is if "COPY" is anywhere on the coin, if "COPY" is stamped in the coin, you can know for sure it is a fake. You might also want to look for a box that looks like its been filed away that had "COPY" stamped on it at one time until one unscrupulous person filed it off. The next thing to do is look marks of where it might have been cast. If the coin looks like it has a seam around it, that is also a dead giveaway it is a fake, real coins are struck, not cast. Next, make sure that the design elements match up with what the real coin is supposed to look like. Now, keep in mind that the dies were hand engraved in 1795, so there are die varieties so your coin might not look 100% like the picture but it should match up closely, but something that is obviously wrong (such as the Seated Liberty design) is enough to tell you that you have a fake. Next, weigh and measure your coin against the dimensions that the coin was produced with. While, naturally, in circulation the coin might change in weight a bit, an incredibly overweight or underweight coin is enough to tell you that you have a rather poor fake. If you still think your coin is genuine, take it in to a reputable coin dealer, he should be able to tell you within 5 minutes if your coin is genuine or not. Keep in mind that to the untrained eye, things like wear and age can be misleading, though an experienced person who deals with coins can easily tell you if it is artificial wear and aging.