Thanks @Raj it might sound silly but I hadn't thought of actually contacting LG or authorised repair centers! haha Well, LG support suggests contacting "Reliable Parts"
https://www.reliableparts.com/ The local branch quoted over $100 for a guaranteed genuine replacement part... so I think I'll pass on that. They also mentioned I could chance it on something from ebay/amazon although then there's no guarantee of the source but it would be ~10% of the cost. The ifixit item looks right, but it ships only in USA.
The phone's model is LG H812, battery says BL-51Y5, the original part number is EAC62818401, Reliable Parts said that is now discontinued and that they would order part number EAC62858501.
Back to the original question, I guess short of trusting the reputation of the seller/store, there isn't really a way to check against fakes or bad products. Reviews of course can be fakes and usually there is a bimodal distribution of them with some 5/5s and some 1/5s... Not confidence inspiring. Short of advanced checking I guess there's after-purchase verification of the battery, but I'm not sure I have the means to do that systematically.
Aside, for 18650, I have the 4 cell battery pack from a dead laptop. I've kept it to salvage the cells though I don't know how much life they have left. Probably for flashlight use. I'd also need to get a charger though.