In Liferay we can Validating the user submitted form or Validating AUI input fields. You have to add aui:validator tag and specify the appropriate attributes which we define with name property for ex: required,minLength etc . The Below Example can show how you can validate the input fields in aui forms.

Ex:

<%@ taglib uri="http://java.sun.com/portlet_2_0" prefix="portlet" %>
<%@ page import="javax.portlet.PortletPreferences" %>
<%@ taglib uri="http://liferay.com/tld/aui" prefix="aui" %>

<portlet:defineObjects />

<portlet:actionURL name="loginCheck" var="loginURL" />

<aui:form name="loginCheck" method="POST" action="<%=loginURL.toString()  %>" class="dropzone" enctype="multipart/form-data">
    <aui:fieldset>
        <aui:input name="name" >
             <aui:validator name="required" errorMessage="Please enter your name." />
             <aui:validator name="minLength">6</aui:validator>
        </aui:input>

        <aui:input name="email" >
            <aui:validator name="email" />
             <aui:validator name="required"/>
        </aui:input>

        <aui:input id="message" type="textarea" name="message">
             <aui:validator name="required" errorMessage="Please enter a message." />
        </aui:input>
    </aui:fieldset>
    <aui:input type="file" name="file" >
        <aui:validator name="acceptFiles">'jpg'</aui:validator>
    </aui:input>

    <aui:button type="submit" value="addDetails" />
</aui:form>