nawk 'NR==FNR{a[$1];next} {if($1 in a) print $1,"Found" else print}'  OFS="," File_B File_A

awk 'FNR==NR{a[$1]++;next}{if($1 in a)print $1,"Found"; else printf("%s%s\n", $1,OFS)}' OFS="," B A

awk 'NR == FNR { _[$1]; next }
($2 = $1 in _ ? "Found" : x) || 1
' OFS=, file_b file_a

#!/usr/bin/env python
file2=[i.split()[0] for i in open("file2").read().split("\n")]
for line in open("file1"):
    line=line.strip().split()
    if line[0] in file2:
        print line[0]," found"
    else:
        print line[0],","

perl -ne 'BEGIN{open(F,"fileb"); while(<F>){split;$found{$_[0]}="Found"} close(F)} {chomp; print "$_,$found{$_}\n"}' filea