Depends on how you want to check.
You do it manually by opening SSMS (Sql Server Management Studio), connecting to your instance (via the first log-in screen) and looking in the Databases list on the left hand side.
Or you can do it in code by listing all the databases on your server instance (if you have a connection string):
using (SqlConnection con = new SqlConnection(strConnect))
{
con.Open();
using (SqlCommand cmd = new SqlCommand("sp_databases", con))
{
cmd.CommandType = CommandType.StoredProcedure;
SqlDataReader read = cmd.ExecuteReader();
while (read.Read())
{
Console.WriteLine((string)read["DATABASE_NAME"]);
}
}
}