how to tell if heater core is cracked or clogged up.

If coolant is leaking onto the floor, the heater core is leaking. If it was just clogged up, it wouldn't leak.

so can some stop leak work

Stop leak is a temporary fix at best. At worst, it clogs up everything making a bad situation even worse.

As a temporary fix, maybe. You need to have the heater core replaced. If you cannot get it done quickly and the leak is relatively small, you can keep adding coolant but that will just leave you with anti-freeze soaking your carpets and the floor pan below it.

Replace the core if you plan on keeping the vehicle.

Don't use a stop leak product!! If you can't get the heater core replaced right now, bypass it by connecting the two heater hoses together! You won't have any heat but using a stop leak product will give you the same result, plus a clogged up cooling system!! Hope that helps! Jim