I can't tell (clause)

In this case, "tell" doesn't mean "say". It means "decide" or "figure out". The phrase "I can't tell ___" is used to talk about something you're not sure about:

I can't tell whether it's finished or not. I can't tell whether she's being shy or just doesn't want to talk to me.

You can use "can't tell" with any question word like "how", "what", "who", and so on:

I couldn't tell what they were saying.

For the life of me, I can not tell how he did it.

what (something) is supposed to (do/be)

One of the ways to use the phrase "supposed to ___" is to talk about what a person intended, or what they were trying to communicate. 

For example, if you see a number written down, but it's not clear, you can ask:

Is this supposed to be an 8?

Or if your friend says something that might be critical of you, you can angrily ask:

What's that supposed to mean?

This means something like "Why did you say that?"

Any ideas?

This sentence is a shortened version of "Do you have any ideas?" Ask this when you want someone to help you think of something like:

• a solution for a problem
• the answer to a confusing question
• suggestions for fun things to do

Here's an example:

We need to get Trey somethng for his birthday. Any ideas?