Aside from searching for the P waves to decipher rhythms, the QRS morphology can give us clues to the rhythm.
This strip is from a patient who was admitted due to abdominal pain. While on telemetry, I noticed this interesting teaching strip.
This is a regular narrow complex QRS rhythm. Starting from the left side of the screen, the P wave is seen to be merging with the QRs until it cannot be seen and reemerged before QRS #7.
Take a look at the QRS morphology/shapes in all leads. You will notice that there are 2 different QRS morphology. QRS #1-6 have the same shape and QRS #7-10 have the same morphology. This means that with a P wave at a conductible distance from the QRS, QRS #7-10 are sinus beats or the ventricles were depolarized by sinus beats.
QRS# 1-6 with some of the P waves fusing with the QRS are junctional beats or the ventricles were depolarized by junctional impulses.
You might also argue that QRS #1 should be a sinus beat because of the obvious P wave and a normal PRI. The argument against it is the similarity of morphology with the obvious junctional beats (QRS #2-6).
Figure - ladder diagram
This strip is best understood using a ladder diagram.
This is a nice teaching strip showing competition of 2 pacemakers - junction and sinoatrial node (SAN). The clue to the diagnosis which captured/depolarized the ventricles was revealed by the QRS morphology.
In this case, the atria was still depolarized by the SAN and it somewhat disappeared because the P waves were "buried" in the QRS. This is called isorhythmic AV dissociation - rhythms with atrial rates and ventricular rates that are almost the same and there is dissociation because the ventricles were controlled by competing pacemaker.