Question 

How can you tell if you have a marijuana seedling (plant), I've never grown one before and need to know what they look like???

Answer

If you didn't plant the seeds yourself then you will have to wait until the plants develop a bit more. Below are some pictures that may help for comparison. Marijuana seedlings tend to have a first set of leaves that are rounded and then all the rest of the leaves tend to have jagged edges on them.

Click on pictures below for enlargements.