Some foals may have grey eyelashes at birth that are going to grey out. However, some foals will have grey or white eyelashes without grey - silver for example is often characterised by white eyelashes. Also, foals with white on their face near their eye, or over it, can have white eyelashes too.
There are other indicators of grey in foals - the first is that one of the parents MUST be grey for the foal to have it. Then it comes down to interpreting the signs on the foal. Grey foals are usually born a more mature shade of their base colour, rather than in foal colours. They may have white hairs already present, usually on the face, maybe in the form of goggles. They usually have dark legs, compared to the usual paler legs that most foals have.
This Arabian foal is a good example. You can see a much more mature black colour, and clear goggles around the eyes.
This foal is a good example of typical foal colouring, and will mature to be a black horse. You can see a general silver tone to the coat, and a definitely paleness to the underside of the body and legs that the Arabian foal above definitely lacks.
When Apache was a baby we had no idea what his parents looked like since he came through an orphan foal rescue. I had no idea about the adult color vs. foal coat color at the time - but he was a very vibrant chestnut tobiano.
About 2-3 weeks after we had him I started noticing a couple white eye lashes and while he had facial markings they were no where near his eyes. I even commented to my vet I thought he was going to go gray and my vet scoffed at it. Now looking back I can see faint "goggles" around his eyes in his foal pics, but only because I really look for them. They were no where near as distinct as some of the foals I've seen, but those couple white eye lashes sure stood out.
Well now at 8 years of age that boy is almost at the end of the graying process and only has a very slight amount of gray dappling left on his chest and flanks.
Not all gray foals have goggles or white hair in the eyelashes. My baby didn't. But he's definitely going gray. He did have the more mature body color. Just no eyelashes or goggles. I think the first hint of white coming in on him was in his tail. Then on his face but on the bridge of is nose. No goggles at all and no white eyelashes until he was probably 2 years old. Here he is as a newborn and now at age 3.