It's hard to say for ~absolute~ certain for a couple of reasons.

1. Our scholarship around Sappho changes very much depending on what is considered "taboo" or "popular" at the time.
2. Sappho's poems themselves are clearly homo erotic, but her exact relationship to the women about which she writes is unclear in no small part because of the next point,
3. "Lesbian" as an adjective meaning a women who likes to have sex exclusively with women is a modern term that, even if we could say for sure that Sappho liked women, wouldn't accurately describe Sappho.

I'll tackle these points one at a time.

When scholarship of Sappho first became huge in the late 19th and early 20th century, the dominant theory about why she was writing about women was that she was a schoolmistress. A bombshell 1993 article by Hold N. Parker dismissed this, saying that the scholars who assigned that role to Sappho did so not because they had hard evidence for it in her poetry, but because they were scandalized by and couldn't really accept the notion of "lesbianism," and so explained it away. Similarly, in a direct refutation to scholarship such as that, classicists of more recent years have been pushing hard in the opposite direction to find as much gay-ness as possible in the ancient world. This is academically more honest than making up a job for Sappho so you don't have to write "lesbian" in your academic paper, but it's good to keep in mind that in 100 more years, there will be a different predominant theory about Sappho's sexuality, so keep an open mind, and look at the evidence yourself.

Parker's article in Chicago style: Parker, Holt N. "Sappho Schoolmistress." Transactions of the American Philological Association (1974-) 123 (1993): 309-51.

I found it on JSTOR, so if you want to pay for it, you absolutely can.

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To move to Sappho's poems specifically, a lot of them are clearly speaking about women in a very erotic way, there's no way around this. Consider poem 1 as translated by Edwin Marion Cox:

I

Peer of the gods, the happiest man I seemSitting before thee, rapt at thy sight, hearingThy soft laughter and thy voice most gentle,⁠Speaking so sweetly.

II

Then in my bosom my heart wildly flutters,And, when on thee I gaze never so little,Bereft am I of all power of utterance,⁠My tongue is useless.

III

There rushes at once through my flesh tingling fire,My eyes are deprived of all power of vision,My ears hear nothing but sounds of winds roaring,⁠And all is blackness.

IV

Down courses in streams the sweat of emotion,A dread trembling o’erwhelms me, paler am IThan dried grass in autumn, and in my madness⁠Dead I seem almost.

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Sappho is pretty clearly in love with whoever she is speaking to in this poem. Is it for sure a women? As a classicist I can tell you "probably," especially given the way the object of her affection's voice is described in the first stanza.

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If you go to this website:

https://en.wikisource.org/wiki/The_Poems_of_Sappho/Chapter_3#P8

you'll find a lot of fragments of Sappho which are clearly erotic, and a lot which are clearly about women. If you really don't want to believe that Sappho had homoerotic desires, you can refuse to see it, it's slightly ambiguous, but you can also put two and two together here.

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Finally, I have to throw my personal pet peeve into the ring here: I, and many other scholars, do not believe that sexuality had been constructed as an identifier or "type" in 600 BC. When you say "Lesbian" today, you do not think of just women with women, you think of Denim jackets and Subarus as well. There is more to a sexual identity than just the sex, to paraphrase Foucault, modern society views homosexuality as a sort of "hermaphrodism of the soul," and ancient people simply didn't. For them, sex was like coke and Pepsi. Most people had a preference, but almost everyone drinks both, and you are even considered a little weird if you are brand loyal to the point of saying "Pepsi is NOT ok."

This is all to say that to call Sappho "lesbian" is to apply a modern sub-culture to a women who would not have fit in at all with "Lesbians" today. It's ironic, because Lesbians were named after Lesbos, where Sappho lived, but it's true. Sappho wasn't a Lesbian. She liked women, and all but certainly had sex with women but she could not have been Lesbian because she pre-dated that scene by 2500 years.

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edits: spelling and grammar.