I've heard NBomes are only active sublingually or nasally so the "if it's bitter it's a spitter" rule surely doesn't reply to NBomes because if you taste it you will probably trip?
Should I taste the blotter on my tongue first or just neck half (if half doesn't work I can probably assume its an NBome because its not active orally.
I don't really have time to get a test kit, can you buy them at head shops?
Also aren't NBome blotters usually bigger and thicker?
The DOx substances and LSD itself are active nasally and sublingually/buccally too.
The fact is though, if you taste it and it's bitter, and remove it from your mouth immediately, you will have absorbed very little substance, and unless the blotters are incredibly potent, far far far far from enough to even remotely be close to tripping

To fully absorb a substance sublingually/buccally you need to keep it there for a long time, probably looking at a minimum of an hour (though 15 minutes is usually enough to get a trip going) - the difference is that with something like the NBOMes, if you swallow your hit after 15 minutes of keeping it in your mouth, it won't be as effective as keeping it in your mouth for longer, since it's not orally active - while with LSD and the DOxes, LSD and DOxes are also active orally, so it doesn't matter if you swallow them too quick, you'll still get the full effects either way.
I've heard several reports that NBOMes flag up as pink (the same colour as LSD) in the Ehrlichs (LSD specific) test kits, I'm not sure whether or not this is true, and it certainly sounds odd to me, but if it is, then even a test kit wouldn't be reliable at this stage, unless you bought multiple test kits to cross-test it. (i.e. Ehrlichs + Marquis + Mecke) - it's worth noting this is just what I've heard from a few people though, it's possible they did not receive NBOMes at all, but due to the possibility being there that this is a genuine false positive caused by the NBOMes, I wouldn't assume a single Ehrlichs test to reliably prove you have LSD.
The simplest method is the taste test, LSD should not taste noticably bitter, or anything other than small taste of cardboard/paper and maybe ink.
NBOMe blotters are not usually bigger and thicker, I've received one that was an entire inch by inch and so 4x bigger than a usual LSD blotter, I've also received 1mg 25I-NBOMe blotters that were the standard 1/4" x 1/4" size that LSD blotter comes in. I also have a 25C-NBOMe hit which is 1mg, and once again 1/4" by 1/4" in size. As far as I'm aware even the smallest hits can fit at least a couple of milligrams, but I may be wrong - it's clear though that the idea that other substances must come on bigger blotter is certainly a myth.
Certainly taste the blotter on your tongue first. If it has a noticeable bitter taste then I'd assume it's not LSD and spit it out and try and ascertain from the dealer what it really was.