One of the easiest things that you can pick up about the strength of your opponents’ hand comes from his raise sizing preflop. Often times recreational players are totally unbalanced when it comes to this concept. In a $5-$5 game maybe they raise to $20 with a hand like AQ and $40 or more with a pair or very premium holding. This stems from them not wanting to get drawn out on with their made hands just like when they size there bets to protect against bad beats post flop.
A lot of times you will see easy pickings when several people have limped in and a player makes a small raise over the top. Let us say, for example, in a $5-$10 game three people limp in and the highjack raises to $40. It is almost guaranteed that this is a pot sweetener raise with a pocket pair or some sort of suited connector. Nine times out of ten when you reraise you will take down the pot. You can also reverse this concept upon an aggressive player to your left. Let us say that you think someone will bounce on your small sizing, a few people limp in and you only make it $35 in the HJ with AA. With aggressive players next to act, especially in the blinds it is not uncommon that you will be squeezed.
Now, this is not to say that you should always keep your preflop raise size 100% consistent. Sometime changing up your amounts is definitely the best play given the situation. On super lose table I may raise twice as much as I normally would because players will call larger sizings. At a nittier table I might raise smaller so that I can get action from worse. Adapting to table dynamics is the reason to change preflop raise sizes not hand strength.