How can I tell if my preschool child has a learning disability?

First of all, caution must be advised.  Learning disabilities (LD) are extremely difficult to identify in preschool children due to many symptoms of LD being a part of natural childhood development.  Even typical children will develop their skills in spurts, advancing with some skills early, others late and still others within the realm of "typical development."  Identification is complicated due to these factors.  Often a parent that "suspects" a disability in their child may base their concerns upon the development of a sibling.  The danger with this is we are under the assumption that the sibling's developmental milestones occurred within the realm of "typical development" and this may or may not be the case.  A parent's best decision would be to bring their concerns to the attention of their school district.  Additional advice would be to do some research regarding developmental milestones and typical age mastery of skills.  Finally, simply observe your child and document what you see to ensure you do not forget what behaviors are the targets of your concern.