Couple of different ways you can have your heart checked for different things.
First and simplest is an ECG. Any GP can do that in their office. That looks for abnormalities in the electrical system of your heart. For someone using gear, the worry with an ECG is atherosclerosis causing blocked cardiac arteries leading to chest pain and possibly heart attacks in the long term. Cheap and easy though, no reason not to ask your doctor to do one, especially if you've been feeling suddenly dizzy or missing beats or anything.
Second is a stress ECG. You normally have to go to a large hospital or a cardiologists for that because no-one else can be fucked having a bike/treadmill taking up space and doing the tests themselves, they take a while. This one is usually done for people who have some chest pain, but no changes on the basic ECG. You do it to find some changes to see if you go on to the third test. Probably not an important one for you.
Third is an angiogram. This one is only done in major hospitals in a specialised theatre, usually by cardiologists. Catheter gets put in through the femoral artery, goes up to the heart, you pump in dye and have a look at the arteries under x-ray. Lets you find blockages very accurately. No reason for you to go anywhere near this without serious symptoms.
Fourth is an echocardiogram. This is a specialised ultrasound of the heart that looks at the shape, size, flow of blood and how the valves are working. This one is another important one for guys on gear to have, as it gives you an exact measure of how much of the blood that's getting into your heart is going on into your body (ejection fraction). That's the major worry with androgen use and hearts (apart from the atherosclerosis). Secondarily to that, it will let them have a look at your valves, which can get stretched and start being shit. Much harder to get done though. Anyone who understands steroids will be happy to send you for one and most GPs should do it whether they understand steroid use or not. If they don't, go to another one. Important test for you to have done, even if you don't have symptoms at the moment. At the very least, you want to see what it looks like now so that in 5 years time (assuming the result is good, less if it's bad) you can get it done again and see any changes or deterioration by comparison.
There's some other ones like stress echos and technetium/thallium scanning but really, the two you should be worrying about are ECG and echo. From memory, a set of guidelines for steroid users I read once suggested ECG every 6 months-2 years and echos every 5, but I'm not sure of those.