I try to ask the following questions:
1. Do they smoke or have pets. The worst collection is the dirty, nasty bucket that you have to dig through. I still buy with both of those but it gives me clues as to what the extent of the damage will likely be, clumps of nasty dog hair or fine little white cat hair in the bottom of the tub.
2. Look for Mini Figures Just watch out for mixed lots. Yesterday I bought a nice tub. Probably 30lbs for $60. Looked like some great Star Wars and Indiana Jones figures. They were all there but no sets. The tub was def a mixed lot. Like large pieces that should have mates but are stand alone. I'll still double my money but it won't be amazing.
3. Look at the colors. If you see the tons of primary colors (especially bricks) it's probably going to have a lot of bucket-O-bricks type stuff. If you see tons of blacks and grey with splashes of color I'd probably think Star Wars. If you see lots of earth tones probably themes like Kingdoms, HP, POP, Indiana Jones, etc. Most collections will have a mix of stuff but after a while you develop a 6th sense as to what will probably be in the collection.