How can you tell the difference between spider and mosquito bites?

This is difficult: This is hard to do without experience and training. Most insect bites are benign and will appear as red bumps, swollen, warm to the touch but not painful (except for the initial bite). Some bites or stings can be quite worrisome such as black widow or recluse bites (and in the southwest desert - scorpion stings) and some can some a total body allergic reaction. These need to be seen asap.