<a>
    <b>zyx</b>
    <b>wvu</b>
    <b>tsr</b>
    <b>qpo</b>
</a>

a/b[.='tsr']

a/b[.='tsr']/position()

int position = doc.SelectNodes("a/b[.='tsr']/preceding-sibling::b").Count + 1;

// Check the node actually exists
if (position > 1 || doc.SelectSingleNode("a/b[.='tsr']") != null)
{
    Console.WriteLine("Found at position = {0}", position);
}

count(a/b[.='tsr']/preceding-sibling::*)+1.

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:output method="xml" encoding="utf-8" indent="yes"
              omit-xml-declaration="yes"/>
  <xsl:template match="a/*[text()='tsr']">
    <xsl:number value-of="position()"/>
  </xsl:template>
  <xsl:template match="text()"/>
</xsl:stylesheet>

count(a/b[.='tsr']/preceding::a)+1.

count(a/b[.='tsr']/preceding::*)+1.

using System;
using System.Xml;

public class XpathFinder
{
    public static void Main(string[] args)
    {
        XmlDocument xmldoc = new XmlDocument();
        xmldoc.Load(args[0]);
        foreach ( XmlNode xn in xmldoc.SelectNodes(args[1]) )
        {
            for (int i = 0; i < xn.ParentNode.ChildNodes.Count; i++)
            {
                if ( xn.ParentNode.ChildNodes[i].Equals( xn ) )
                {
                    Console.Out.WriteLine( i );
                    break;
                }
            }
        }
    }
}

count(a/b[.='tsr']/preceding-sibling::*)+number(boolean(a/b[.='tsr']))

index-of(//b, //b[.='tsr'])

count(attr/value[.='$TARGET']/preceding-sibling::*)+1

count(attr/value[.='$TARGET']/preceding::*) + 1