There's not much you can do to tell from the feel of the high.
When I first started, I did the cleanest pills my friends could get me - I knew that feeling, it felt like I was light and the happiest person ever. Then I got my hands on some more, but of a different kind and I got an even CLEANER feeling, only to test it and find out that it was positive for meth/amphetamines.
You see, the "clean" high you get off of pills (very light feeling, amazing body feeling) - Speed also gives you a feeling similar to that, but speed is a lot more harmful to your body than MDMA.
I find - the cleaner they feel, the dirtier they are. I don't know why - where I live, the whole scene is just fucked.
However if you have a pill and you feel out of it, throwing up, angry, depressed, or just over-all shitty, they're most likely dirty.
A clean pill will mainly make you really relaxed, you'll be very emotional(a lot of the times, at least), but if you're on enough clean pills it starts to feel like speedy pills almost. I don't know how to explain it.
IN CONCLUSION:
Don't base it on how the pill felt, TEST IT!
It's the only way to be positive on what you're taking, and it's the safest way next to not doing it at all.
Have fun, and be safe.