To check for heartworm they will take just a drop of blood and look at it under a scope. I buy Vegas's Heartgaurd Plus at the vets, it is givin once a month and is a block of meds that are made like a treat. He loves them.
The box I get has 6 doses and had 6 red hearts that you put on the day of the month on your calender to remind yourself it is the day to give them one. It is VERY important here in Florida since it does not get cold enough to kill mosquitoes, we have them all year long, so Vegas has always been on them.
If you have mosquitoes it is very important that they get heartworm pills.