Shoot a ruler and check the width your image can cover. Divide the width of your sensor to that, you get the macro magnification of the lens. For instance, APS-C sensor size: 22.3x14.9 mm, the width (on ruler) you capture is 22.3 mm, then it's 1:1 (22.3:22.3), if it's 11.15 mm then it's 2:1 (22.3:11.15).
Did some tests this evening using the ruler setup:
I used a Nikon D7000 and a SB600 flash in remote for lighting. I set the flash at -2EV and did not bother to tweak it... so some of the photos underexposed. (And sorry... the ruler is old and dirty...)
Baseline setup: Micro-Nikkor 105mm at 1:1. The focusing distance was 15cm. The ruler says its 24mm - though the Nikon APS-C sensor is suppose to be 23.1mm... may be I am the lucky one who got a bigger sensor for free Nah... more likely, the ruler was not completely perpenticular. Anyway, I will take 23.1mm as the baseline.
Setup 2: Micro-Nikkor 105mm + 12mm Kenko Extension Tube. Measured 20mm, magnification 1.15x, focusing distance 14cm (there was some difficulty in measuring the focusing distance... so it is an approximation)
All focusing done via live view... this is the winner of the magnification contest:
The cheapee 18-55mm kit lens reversed mounted using a BR-2A + all three Kenko extension tube in the set + the 2x TC... > 15x.
Really happy with this lens... I bought second hand at $100... it is great for IR photography, and when reverse mounted gives great magnification. However, I must say that at f3.5, the amount of light loss was very significant and it took me more than 10mins to get the ruler lined up (and not quite focused in the end).
Next, 1.4TC + all 3 tubes + Micro-Nikkor 105mm (set at 1:1 distance):
1.15x magnification. Image a bit OOF coz only after the whole thing did I realized that I forgot to turn the VR off, causing some lens motion during the experiments. The good news is that the focusing distance is 11.5cm.
Here's the 100% crop:
Next, 1.4TC + all 3 tubes + reverse mounted 20mm manual focus lens (sorry, my 50mm is 58mm filter size and I only have the 52mm reverse mounting ring - so can't test 50mm)
In conclusion, I should thank commonjunks' posting for triggering me to experiment with this. It was very fun - though I still have much to learn esp. about macro photography. None of this will work on things that move. At a focusing distance of 3cm, getting sufficient light, as well as focusing was an issue. And the irritating VR boo-boo also taught me to be careful the next time. Further, with one extension after another, the setup felt fragile.