Hi
Is your cat obese? It can be harder to feel a heart beat or pulse in a fat cat. If she's not too big, you should be able to feel her femoral pulse. It's in the 'armpit' of her hind leg, on the inner thigh of the hind legs. It's just a pulsating 'string' going down the leg towards the foot so you have to feel around for it sometimes. If a cat is fat, there can be so much fat there that it can't be felt easily.
If you're feeling it in her chest, put your fingertips on the sternum (breastbone) just below the level of the armpit if the cat were vertical. Then slide just a little to the cat's left side of the sternum. You need to leave your fingers there for a few seconds to get used to the movement of the chest as she breathes, then you should start to feel the heart. In a dog, you can often feel it right away but in an adult cat it can take just a moment, esp. if s/he is fat, furry, or very well muscled.
You can get very cheap stethoscopes at many pharmacies, actually -- less than $10 in some cases, so that's an option too. You would put your stethoscope on the chest, over the breastbone but then tilt a little to the left side (the cat's left). You can also order one online and get a decent one for less than $50.
If your vet suspected an arrhythmia, s/he should do an ECG (also called EKG). It's important to know what type of arrhythmia it is. With some or arrhythmias, you can have a difference in what you hear in the stethoscope and what you feel in the pulses if they 'drop beats.' If you're feeling the pulse, it feels like Pulse Pulse Pulse <pause> Pulse etc. or P P P <weak or different-feeling P> P P etc.
