Hi OG! Unfortunately, there isn't an all singing all dancing memory contention self test that I know of. It very much a "Run this Game/Demo/TestTool and check that the things you see on screen when running a real Spectrum are the same as when you are running a spectrum emulator" affair. Also, just having a perfect memory contention implementation does not mean all multi colour games & demos will look perfect. You need perfect ULA emulation too (So that the screen raster is perfectly sychronised with the Z80).
There are a number of ULA test tools that give an insight into the ULA timings of the Spectrum model that it is running on. I've ran all the tests I could find on my actual 48K and 128K+2 (I've posted links to galleries containing screenshots of my results), and have checked these results match perfectly with mame.
The tests can be found here:http://wizard.ae.krakow.pl/~jb/qaop/tests.html
If you wanted a more enjoyable place to start, try loading the Nirvana+Engine demo on an 48k, 128K or +2A/+3. The tap can be found here:https://spectrumcomputing.co.uk/index.php?cat=96&id=30002
If contended memory and ULA timings are correct, then the multi colour graphics will display correctly (I.e. When the demo runs you should see more than 2 colours occuyping an 8x8 pixel block, without any flicker or colour clash) regardless of what spectrum model you run it on.
Hope this helps