How to tell if a file is a Bitcoin wallet?
the wallet.dat file is a database to store the keys. It has a length of several Megabytes, and tends to grow with the number of tx/keys. Make a copy of it onto a USB drive, and put it aside! If you are on a unixoide system, have a look at the tool "hexdump". You could copy the first 5 kilobytes (dd if=xxx.dat of=/tmp/mytest bs=1024 count=5), and then use "hexdump -C /tmp/mytest". If it looks like this:

00000000  00 00 00 00 01 00 00 00  00 00 00 00 62 31 05 00  |............b1..|
00000010  09 00 00 00 00 10 00 00  00 09 00 00 00 00 00 00  |................|
00000020  3f 2b 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |?+..............|
00000030  20 00 00 00 36 2a 93 00  02 00 00 01 1d f3 cd 88  | ...6*..........|
00000040  18 03 00 00 00 00 00 00  00 00 00 00 02 00 00 00  |................|
00000050  00 00 00 00 20 00 00 00  01 00 00 00 00 00 00 00  |.... ...........|
00000060  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00001000  00 00 00 00 01 00 00 00  01 00 00 00 00 00 00 00  |................|
00001010  00 00 00 00 02 00 f0 0f  01 05 f8 0f f0 0f 00 00  |................|
00001020  01 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00001030  20 00 00 00 36 2a 93 00  02 00 00 01 1d f3 cd 88  | ...6*..........|
00001040  18 03 00 00 00 00 00 00  00 00 00 00 02 00 00 00  |................|
00001050  00 00 00 00 20 00 00 00  01 00 00 00 00 00 00 00  |.... ...........|
00001060  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00001ff0  04 00 01 00 00 00 02 00  04 00 01 6d 61 69 6e 00  |...........main.|
00002000  00 00 00 00 01 00 00 00  02 00 00 00 62 31 05 00  |............b1..|
00002010  09 00 00 00 00 10 00 00  00 09 00 00 00 00 00 00  |................|
00002020  02 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00002030  20 00 00 00 36 2a 93 00  02 00 00 01 1d f3 cd 88  | ...6*..........|
00002040  18 03 00 00 00 00 00 00  00 00 00 00 02 00 00 00  |................|
00002050  00 00 00 00 20 00 00 00  03 00 00 00 00 00 00 00  |.... ...........|
00002060  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00002200  c7 76 7b 67 ff 4d fe 20  f0 4f ef 1f 98 be f0 7c  |.v{g.M. .O.....||


then indeed there is a high possibility for your wallet file.
If you are on windows, there are freeware tools to hexdump or hexedit files, you may not even need the command line.