If you're new to Ukrainian and find that out in the world on social media or otherwise you can't tell if you're reading Ukrainian or Russian, here are some tips:
These letters are typically or exclusively Russian:
ё, ъ, ы, э.
If ё is used, I haven't yet seen it, and may only be emphasized with foreign words.
э is the reverse-elliptical E. In Ukrainian it's є.
The giveaway letter for me every single time is the use of ы. It's not used in Ukrainian, as the letter и is used instead. Because of the "i" sound (as in the word "ill"), it is such a common sound for those letters to be used in many words. It's hard to read any amount of text without spotting either ы or и. If you see one or the other, these two letters will be the giveaway of which language you're reading.
Then there is the problem when it comes to short text, for example on Twitter or Instagram, and someone doesn't use those letters when saying a few words. Look for other clues like key words often used in social media -- in Russian such as: нет (no), да (yes), Спасибо (thanks), as opposed to Ukrainian: ні (no), так (yes), дякую (thanks).
Aside from those different letters, and those three words, for someone at A1 or A2, may have difficulties knowing whether you don't know the word yet (thinking it might be Ukrainian), or if it's a Russian word and shouldn't feel so bad not knowing. Improving vocabulary is going to be critical to know the difference between the words, but also understand that many words are indeed shared or incredibly similar.