I got frustrated with the 'pay for first three chapters' model some apps use now, so I hunted around. Honestly, the most reliable method I've found is going directly to the publisher's or official translation site's listing page. Most of them have a 'read free sample' button right there, and it often includes more than just chapter one. I read a bunch of 'Doomsday Wonderland' that way on the host site before deciding.
Another trick is that some fan translation sites still post early chapters to draw you in, but the quality can be all over the place. I'll use those for a taste, but if I like it, I try to find where the official version is. My rule is if the preview hooks me and the official translation is ongoing, I'll follow it there. Saves a lot of disappointment later when a fan project drops it halfway.