The Limits.getLimitEmailInvocations() call should give you what you need. More info on limits here

UPDATED ANSWER:

Turns out, the Limits class is now what you need. I'll go back to what I used to do.

You can use Messaging.reserveMassEmailCapacity(count);. This call will result in an error if you don't have enough capacity. It's not exactly what you asked, but it could do the trick in your case.

Reference: https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_forcecom_email_outbound.htm

Also, vote the idea that would expose this limit via the class above: https://success.salesforce.com/ideaView?id=08730000000aSLLAA2