If you are not part of the company, the information can only be obtained by direct communication with the client. Unless this is a company in which prints out quarterly reports, in which you can view and see if there has been much of a change in their business!

Are you concerned with how your mailer effected people to look into the product!?

Do you ask the client to keep you informed, or contact the client later on?

Both

"Are you concerned with how your mailer effected people to look into the product!?"

I am not concerned right now, but I just wanted to know how other designers went about this to see if I had the right idea.

My guess would just to be ask the client. They will tell you if their sales increased as a result.

For this to work you would have a good relationship with the client. Otherwise, they won't answer the question with honesty. That is why I say, it is best if you are part of the company - then your concern seems to have the best interest in future designs for the client.

I have a few on going clients and to me it seems like I work for them. I have an office in two of the clients sites that I rarely go to, but they like to have it just incase it is time for producing and meeting, producing and meeting!! I think you get the point.

Another way you could tell withoug actually getting numbers is if they call on you again. If you get another project from that client, odds are that is a sign that what you did worked and they want you to do it again. When they call you back for future projects, ask "how did the other flyer/mailer do?".....