I've long been a fan of the excellent Andean Travel Web but have only just discovered their facility for checking permit availability in real time. (The official Peruvian government website is very difficult to use, and I don't just mean because it's in Spanish.) Simply visit this page
and click on the month you are thinking about. For example, clicking today (21 June) you can see at a glance that there are zero permits available on any day throughout July to September, that there are some in October, especially in its second half and that only November and December have any days in which the full 500 permits appear to be available.
Bear in mind that the apparent limit of 500 includes trekking staff who outnumber visitors, so about 200 trekkers per day can start on an Inca Trail (and they are split among the classic 4-day trail, the short 2-day trail and the 7-day Mollepata trails). Even if companies have cancellations, they are not allowed to fill these spaces with other clients (a sensible precaution against abuse). Also, as soon as some permits start to sell on a specific, there is a domino effect whereby other companies buy permits to avoid being left stranded, so the number available can fall very fast.
This is a major time-saver, because you can check the months so quickly before even talking to tour operators. No permits means there is no point in phoning around: your choice is simple, you either pick another date or choose another trail. And the Andean Web Travel site has good advice to offer about choosing an alternative trek