by testing you mean running a restore after you've made a backup
In the ideal situation you would be fairly close to "factory condition" so that you didn't lose too much if it didn't work. My XP system got hosed due to virus so I took the opportunity to install XP Pro. I researched the image backup programs around and bought one. I installed a 1/2 dozen or so small applications I was very familiar with so that when I did the restore I'd sense it if stuff was weird.
Ok, I did the image backup with the program I bought. Then I booted a partitioning CD and deleted my C: partition.
I booted up the Restore CD from the image program and restored the partition from the backup. Everything seemed fine. Since then I have tried out W7 beta and when done playing, just restored my XP back to where it was.
The trick is research. Go on the program's forum and see if people with configurations like yours just can't seem to get the thing to work right. Every product will have a few unlucky customers but if you read a bit you can get a good idea if the program will work well with your setup(unless you have some home built PC then only you would know.)
The one I got was Paragon Drive Backup Personal. Since then I bought a PC with Vista 64 bit and got their free Express 64 bit version.
You can have more confidence if you know the thing works by trying it.
If you're going to try it, better to do it before you have 2094 programs installed.