Why did Walt suspect Krazy 8 of hiding a shard of the broken plate? What made him suspicious, sort through the garbage, and put together all the broken pieces of the plate he'd dropped?
Walt had killed Krazy-8's cousin Emilio, and had all but killed Krazy-8 with the phosphine gas. However Walt was not at this point a ruthless killer, he had improvised the gas as a way of escaping from what was increasingly looking like a fatal situation for Jesse and himself.
He was now however faced with the job of killing Krazy-8 in cold blood, and was trying to rationalize his way out of the job, and was befriending him with sandwiches & beer in an attempt to find some means however slim of having to get out of the job of killing him. Walt is not particularly in a rush either, and Krazy-8 is not in a position to escape.
However Walt knows this is mostly procrastination, and that it is unlikely that some reasonable path out of this can be found. We've seen since that Walt is not stupid and knows that he needs a watertight reason to trust some of the people he deals with. He would only not kill Krazy-8 if he had some leverage to ensure his and his family's safety.
Walt has been very careful with how he deals with Krazy-8 - such as to not put himself in danger. The time he passes out and breaks the plate is the only time his guard is dropped and he knows it. I think Walt is thinking through all the possible consequences of the event, and it occurs to Walt as he looks at the plate in the trash that he needs to know that he has all the major pieces.
Walt's strength is his analytical mind. I think that this is just one of many examples of Walt thinking through all the possible consequences.
I don't believe Walt suspected him of doing this at all. Remember he was going up stairs to get the key to let him go. Just by pure chance he went to throw his can away and sees the broken pieces of the plate. At that point Walt's analytical mind realized that those pieces of the plate don't add up to a whole plate. He attempts to build the "plate puzzle" and shockingly realizes his mistake in trusting this guy.
If he had suspected him of some underhanded act, he would not have went to get the key and he would not have been so upset after finding the missing plate shard.