Swollen lymph nodes are usually palpable and may be tender to touch. Any particular reason why you're concerned? How big were the ones that you felt under your arm?
Lympyh nodes are best assessed by palpation rather than visual inspection (unless it is of significant size) and then if it's suspicious, a biopsy may be needed.
Some lymph nodes are arranged in groups to drain lymph fluid from specific areas of the body. Some of these are found in the neck, behind the ears, armpit, inguinal area (singit), popliteal area (back of the knees), etc. You may have noticed your doctor palpating these areas during a routine medical examination.
In some people, they can be easily palpable while in others they are not. If you're currently sick or just recovering from an infection, in most cases, lymph nodes in that specific area (eg. cough & colds -- neck, near the chin) will be palpable and may be tender to touch but this resolves on its own after a few weeks.
If, however, it doesn't seem to resolve, try to palpate it (if it doesn't hurt) and approximate the size (eg. munggo bean, corn kernel, 25c, etc.). Anything bigger than a 25 centavo coin should be suspicious, especially if with other symptoms. Observe for up to 6 months on whether it changes in size, texture or appearance. If it does, consult your doctor for a full evaluation.