The problem is you can't test that something is safe with your bare eye (e.g. you can't apply 2.2kN to the carabiner in the store, organize a fall, etc..). So you have 2 things to rely on:
- Certificates (UIAA, CE and/or specific to your country)
- Other people's experience
The second option is not so useful with small, not so widespread brands (and the reviews may be biased: a couple of schoolboys can break almost any piece of equipment by using it wrong or praiseful reviews can be written by company affiliates).
At least, you can always tell if something isn't safe: the seam on the harness might look weak or expose threads sticking out all over, the gate of the carabiner might jam or the particular piece of equipment might just feel unsafe. Don't use it in case of any doubt.
P.S. Low price doesn't always imply bad quality, small companies don't charge you for the brand and they need to keep low prices to be competitive.