If you are careful, you can open the bucket and take a look, but you won't see the trub line that blindlemonlars mentioned.
I use the 7.9gal "wine and beer" buckets for primary fermentation and 6.5 gallon glass carboys for 'secondary fermenters'(conditioning).
The buckets are cheaper for me w/ more capacity. The glass is impermeable to oxygen. Better bottles are much safer, though depending on your area may be hard to find or expensive.
If you are going to primary in a carboy, you should also set yourself up some equipment to thief(BTW, should be added to WIKI) AKA, get a sample to test (usually for specific gravity readings).
IMO, if the valve will hold liquid in, it will hold liquid out(probably more likely due to an equalization of hydrostatic pressure when the bucket is submerged.