First off you need to look at where the ad was placed. Where did you see the ad? When you receive these ads in your email, regular mail, or on a poster stapled to a telephone pole you can rest assured it is garbage. Ads placed in legitimate newspapers or trade magazines may be more likely to be legitimate but it is still unlikely. The best defense is to be on guard and assume that any home business offer is a scam until proven otherwise.
One of the biggest work at home scams around is that of envelope stuffing. This one has been around for years. I can remember seeing these ads as a teenager and I am 45 today! What these companies usually do is charge you a fee to become involved and then what they do is send you envelopes and ads to place like the one you answered. Yeah, you guessed it; you are stuffing envelopes and sending information to other suckers like you who got roped into the scam. These types of operations are illegal in almost every jurisdiction. Don't fall for the envelope stuffing scams.
Another way to identify if a home business opportunity is a scam is if they are charging for supplies. Nearly all of these bogus work at home offers will ask you to put up money for supplies that they say are needed to do the work. If anyone asks you to invest in materials it isn't a legitimate offer. A reputable company that is seeking people to work from home will not require you to pay for anything upfront. If there are things needed such as computer software, they will either provide them free or deduct the fees from your earnings.
Another bogus work from home opportunity you need to be aware of is the one where they send you the materials upfront to make items for them such as clothing, toys, crafts, or other items. At first you may think that it is a reputable operation because they didn't charge you for the materials. However, what you quickly find out is that when you send in your completed work, they refuse to pay you because it isn't "up to their standards". If you are going to do this kind of work do it for yourself.
So there you have it. Some of the more common work at home, business opportunity scams. Just remember the old adage, "If it sounds too good to be true, it probably is".