The easiest difference to spot is the ears.
Llamas have long banana-shaped ears while alpacas have shorter spear-shaped ears.

There really are a lot of differences between llamas and alpacas.
The llama is roughly twice the size of the alpaca. Most alpacas weigh between 100 and 175 lbs when fully grown.

Llamas on the other hand weigh in the neighborhood of 200 to 350 lbs. with some as heavy as 400 lbs. and the llama has a very coarse outer coat over a softer inner coat - as opposed to the alpaca, which has a very fine, single coat.
In addition, the llama produces far less fiber per animal than the alpaca, despite its much larger size.

This is because the alpaca was domesticated and carefully bred for over 5000 years as a luxury fiber-producing animal. The llama has been bred for the same amount of time as a pack-carrying animal.