I check and see the spacing of the 8 on 18 to see if is completely in the blue or partially in the red. I also look at the 6 and 8 to see how fat they are.
Look at enough bezels, and can tell pretty easily.
After spending a while looking for a correlation I haven't found any evidence for the blue/red split vis a vis the 1 / 8 in 18 meaning anything. I think this just changed based on the rotation of the red/blue as they put it on the number printing slab. The 6 and 8 'fatness' on super fat font bezels can be see on the late 1960's watches but originally weren't seen on early 60's gilts ... though now as they are popular and therefore more expensive you can see them making their way on to expensive chapter ring gilts. More a product of dealers than anything based in scholarship.