To find cheats without the mame built-in engine (that is not present at the moment in the mame releases) you must:
1) Run mame in debug mode (at the command line or in a batch file write mame.exe -debug or mame32ui.exe -debug if you use the win32 version), choose the game and hit F5 to start it.
2) Then start a software to find cheats under windows (like ArtMoney Pro or many others).
A) with the cheat software find the location of what you want to cheat (es. lives); i suppose you know how to look for cheats with a common cheat software.
B) modify that location to a value you can easely find (ex. DEADBEEF or CCCCCCCC or AAAAAAAA or BBBBBBBB; avoid FFFFFFFF or 11111111 because these are common bytes sequences and you may find wrong locations using those) remembering to set, in the cheat software, the value of the location at least to 4bytes because some found valueas ca<n be 1 or 2 bytes only and changing them to for ex. AAAAAAAA is not possible because you will only obtain AA for 1 byte and AAAA for 2 bytes.
The location you find with a common cheat software is DIFFERENT from the one needed for mame cheats and often it's different also among cheat softwares themselves.
so now you can do 3 things:
C1) go into mame debug window, press CTRL+M to open the memory editor and manually (visually) look for the value you entered (es. DEADBEEF, AAAAAAAA or BBBBBBBB) and write down the mame debug memory editor location: this will be the good address to write in the cheat file. Maybe try to modify the address in the cheat software to verify that the changes correspond into mame debug memory editor so you are sure you found the correct location (because you can also find false positives with AAAAAAAA, BBBBBBBB etc.). Unfortunately some mame cpus use a very big ram (ex PSX hardware) so this method is really time-consuming, use it only for old cpu games (less memory used).
[NOTE: using the byte sequence DEADBEEF it's really a good choice because it's very hard to find this sequence of bytes in games memory but sometimes some mame cpus like to invert bytes order so if you modify the value in the cheat software with DEADBEEF you can find it EDADEBFE in the mame debug memory editor, so using BBBBBBBB or CCCCCCCC is more convenient because inverting those values it always gives the same sequence
but it may create lot of false positives; make your choice according to the game you are "exploring" !]
C2) another way is to use the mame debug find option: it look for a specific sequence of bytes you specify giving you back the addresses in which that sequence is present (not so good in my opinion).
C3) use the mame debug dump option: it creates a file in which you can specify the amount of memory to dump (from 1 byte to all the game memory) and then look with notepad inside it for the specific sequence). It's longer to obtain the memory file dumps in games with big memory but the notepad search is really faster than mame find command.
To know how to use the find and dump command type "help dump" or "help find" (without quotes) in mame debug window, then hit enter: you will receive lot of examples !
When you input commands in the mame debugger windows, if you want to obtain the output of the command you entered the game MUST BE RUNNING (NOT IN PAUSE); you can give a command also when the game is in pause but the output will come out only AFTER the game is unpaused.
Here you can find almost all outputs of the mame debug help command:
http://mess.redump.net/debugger and here in particular the memory find and dump commands syntax:
http://mess.redump.net/debugger:memory
Once you find the correct address you can create your .xml files according to the specific game name and insert it modifying it with the value of your choice (have a look at an already done .xml file to see how to "compose" a cheat.
2 Tricks:
1) if you enter values as something like 44414D4E434F4F4C you will find the correspond ASCII characters (right part of the mame debugger memory editor) saying DAMNCOOL and it will be easier to visually look for DAMNCOOL insted of 44414D4E434F4F4C. Obviously this doesn't work in game cpus which invert bytes (try to invert them before like 4414D4E434F4F4C4).
2) after you found your 1st game value (ex. lives) VERY often other significant memory locations are NEAR the one you found, such as CREDITS, ENERGY or TIMERS; so after you found your 1st value try to VISUALLY look in the nearer memory regions of the mame debugger memory editor; a timer will decrease or increase while the game is running, a byte will change if you get powerups, etc. just use your fantasy to became skilled in finding cheats !
One last note: remember that, in memory editors, values can go on a new line so you can find for ex. AAAAAA at the end of a line the remaining AA will be at the beginning of the line below or you can find CC at the end of a line and the remaining CCCCCC at the beginning of the line below or you can find DEAD at the end of a line and BEEF at the begninning of the line below. Remember that apparently insignificant note when you look for the values you entered (using find, dump, visually) because a simple search of the plain value, ex. CCCCCCCC, can produce zero results for this reason; so try to look also for CCCCCC or DEAD or DEADBE or ADBEEF etc. !
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