If you want to use snoop or tcpdump with advanced search in the packet, then you would need to convert the string to hex string. For this, either you can use a web search and find some web application to do that or you can use a simple C program like this 🙂
/*
* =====================================================================================
*
*       Filename:  ascii.c
*
*    Description:  ascii to dec
*
*        Version:  1.0
*        Ceated:  03/02/2012 12:08:49 PM
*       Revision:  none
*       Compiler:  gcc
*
*         Author:  Amit Agarwal (aka), amit.agarwal@roamware.com
*  Last modified: Thu Mar 08, 2012  08:47PM
*
* =====================================================================================
*/
void main (int argc, char **argv){
    int i=0;
    char a[200]="",b[200]="";
    printf("String - %s, Length - %d\n",argv[1],strlen(argv[1]));
    /*printf("%x\n 0x",argv[1]);*/
    while (i < strlen(argv[1])){
        /*printf("%c",argv[1][i]);*/
        sprintf(a,"%s%x",a,argv[1][i]);
        /*printf("a=%s\n",a);
        printf("b=%s\n",b);*/
        sprintf(b,"%s%d",b,argv[1][i]);
        i++;
    }
    printf ("\n");
    printf ("String : %s\n",argv[1]);
    printf ("Hex    : 0x%s\n", a);
    printf ("Dec    : %s\n", b);
}