You can easily find the files with %20 in the file name just search for that sequence, e.g.
File name: %20
Containing text:
Look in: E:\
Automatically replacing is a little more difficult. Using the XSL Transform below you can generate a batch file to rename each file, removing the %20.
Save the transform below to a file, e.g. rename_file.xsl, and then follow the steps in this QA article to apply the transform to your results and create a text file that can be used as a batch rename script.
<?xml version="1.0"?>
<!-- Rename a file to remove %20 from name
Copyright (C) Mythicsoft Ltd 2015. All rights reserved.
-->
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:rslt="http://www.mythicsoft.com/FileLocator_16Aug2005"
version="1.0">
<xsl:output method="text" indent="yes"/>
<xsl:param name="replace-text">%20</xsl:param>
<xsl:template match="/">
<xsl:apply-templates select="//rslt:file"/>
</xsl:template>
<!-- Rename command -->
<xsl:template match="rslt:file">
<xsl:text>ren "</xsl:text>
<xsl:value-of select="rslt:path"/>
<xsl:value-of select="rslt:name"/>
<xsl:text>" "</xsl:text>
<xsl:call-template name="string-replace-all">
<xsl:with-param name="text" select="rslt:name" />
<xsl:with-param name="replace" select="$replace-text" />
<xsl:with-param name="by">
<xsl:text> </xsl:text> <!-- Replace with space -->
</xsl:with-param>
</xsl:call-template>
<xsl:text>" </xsl:text> <!-- CR LF end of line -->
</xsl:template>
<xsl:template name="string-replace-all">
<xsl:param name="text" />
<xsl:param name="replace" />
<xsl:param name="by" />
<xsl:choose>
<xsl:when test="contains($text, $replace)">
<xsl:value-of select="substring-before($text,$replace)" />
<xsl:value-of select="$by" />
<xsl:call-template name="string-replace-all">
<xsl:with-param name="text"
select="substring-after($text,$replace)" />
<xsl:with-param name="replace" select="$replace" />
<xsl:with-param name="by" select="$by" />
</xsl:call-template>
</xsl:when>
<xsl:otherwise>
<xsl:value-of select="$text" />
</xsl:otherwise>
</xsl:choose>
</xsl:template>
</xsl:stylesheet>
It should produce something like this:
ren "E:\test%20file%20With%20Spaces - Copy.htm" "test file With Spaces - Copy.htm"
ren "E:\test%20file%20With%20Spaces.htm" "test file With Spaces.htm"