How will I know if I have gained muscle?

You will see it. Use a tape measure for the body and place it around your bicep once relaxed and once flexed and write it down, do it every few weeks. U will begin to see a difference.

It's hard to actually "measure" muscle size. You can go to a gym and have them do a "BMI" a body mass index. This will tell you how much of a percentage your body fat is, and how much is muscle.

To build your arms you can do certain exercises. Do curls w/ barbells for your biceps, and other exercises for your triceps. (The two main muscles of your arm) Just make sure you don't don't exercise the same muscles 2 days in a row. Your muscle has to rest a day or two. When you lift weights, you are actually tearing the muscle. The 2 days of your muscle healing is actually the building of the size of your muscle. You should also do the whole body to keep in proportion. (legs, back, shoulders, chest) Just remember muscle weighs more than fat, so if you start gaining weight, and you're exercising, it's most likely muscle weight...

If you want to see the physical difference in your muscle apperence...Take some "before" pictures with a webcam or digital and cate them. Then take the same pictures (same position about once a month and you should be able to see your results...

Here's some basic exercises...Hope it helps...

http://www.ironworkout.com/arm_workout.htm