You don't
need to see in the fermenter. Some folks, including me,
like to see.
How do you know when it's done fermenting if it's in a bucket? I like to know when the yeast drop so I know how much longer I have to wait to move to the next step....dry hop, cold crash, packaging etc. I'm not patient enough to just wait 3 or 4 weeks.
Make a lid from some clear 1/8 or 1/4" Lexan or something. That's what I did. People often freak out because I just place a square of Lexan over the mouth of the carboy. "It doesn't seal!" they shout. "Doom! Horror! Zombie Apocalypse! OMGWTFBBQ!!!!1!!1!!"
Whatevs.
Traditional open fermentation, which I have now practiced for the better part of two decades professionally and on a hobby basis, means you can and will produce excellent beer without dealing with those goddam bucket lids.
Bucket lids suck. First, they never seal reliably; second, they're a complete PITA to remove. That means A., the sanitation argument is invalid, and B., when you try to open the thing to take gravity readings or whatever, you stir up all the gunge which just took a week to flock out.
The ferment will devolve enough CO2 to completely flush the fermenter's headspace with carbon dioxide. The skin of krauesen thrown up will further protect the beer from contamination. So there's no reason to worry about it.
How do you
know it's done? You take consecutive hydrometer readings over the period of 2-3 days.
If you have a tight seal on your bucket and you use a airlock, you can tell by the reduction of bubbles.
GACK! No, no, a thousand times NO! This misinformation needs to be taken out into the street and shot. Say it with me: "Bubbles in the airlock mean NOTHING. The only measure of fermentation is taken with a hydrometer." Now repeat until you drive that bubble crap completely out of your conscious and subconscious.
Cheers!
Bob