It's almost same, but a little different. They are distinguished by tongue's position. But when we talk, it's hard to distinguish.

That's really hard to distinguish between those vowels from pronounciation. 

There's 2 ways to see them differently.

1. make sounds first ㅓ then ㅣ and try to put them together quickly

    same as ㅕ + ㅣ and ㅑ + ㅣ

    also, "ㅓ+ㅣ" makes sound lil bit longer than ㅏ+ㅣ

    same as "ㅕ+ㅣ" makes sound lil bit longer than ㅑ+ㅣ

2. depends on words, we can predict which one is which.

    게 = crab               예 = yes (formal)

    개 = dog                얘 = hey (calling someone)

I am not a professional Korean teacher.

That's just my opinion.