Crumpler:
To check fake or not, visit their official site, if the particular model's design/colour mix is different from their website, then high possibility it's fake. Crumpler is easier to differentiate through pic as fake bag always look soft, not solid, dimension and shape not perfect, distorted. Material much more inferior, padding softer, lousy stitching. Some of the fake look much different than original too, for example Crumpler Jimmy Bo. So it's much safer to look at pic of actual bag before buying.

Another clue is the price. If the price is so much cheaper than UK/USA's price from reputable online shop, then very high possibility it's fake.

Lowepro:
I have friends using fake Lowepro bag (which they don't even realize), 1 of it same model as mine, so i hv the chance to compare it side by side. These are my findings:
1. Fake and original's plastic bag is same.
2. White tag with red circle also same.
3. By just looking at it, it's very hard to tell whether it's fake (unlike some of the crumpler fake)
4. Material is different. Texture almost same, but fake feel much 'synthetic', which have a strong chemical/plastic smell. Original don't have smell.
5. Zip quality is much inferior. Try to zip in fast speed. Original is 100% smooth. Fake can never achieve that. It will get stuck/not as smooth.
6. Belt clip. Original also feel very smooth and fit nicely. You can clip very fast. If you find yourself struggle to attach the clip, then it's fake.
7. Padding insert, almost the same. Very hard to differentiate.

All the fake I came across matched above description. I owned 5 different original Lowepro models, so I can assure you, all the zip/clip is very smooth, even after 4 years of rugged use.

OEM claim is bullshit. They just copy the design 100%, but never use the same material.
Again, compare price with reputable online shop. If it's too cheap to be true, then it's fake. You may ask the seller if the bag is original, some seller is good, they will tell you it's fake, thus the cheap price. So it's up to you to choose, cheap for fake, or expensive for a original good quality bag.