For ventricular rhythms, examine the R to R intervals on the EKG strip. Calipers or paper marks can be used to fix the distance for one R-R interval and then this distance can be compared to other R-R pairs. Within this web site, you will also be able to use virtual calipers.
Are they regular, meaning that each heart beat's R-R interval is equal? Small variations of up to 10% are considered equal. Is the rhythm regularly irregular? For example is there a pattern, such as increasing R-R durations? Or perhaps groups of similar intervals as illustrated on the right? Or are R-R intervals completely irregular?
For atrial rhythm, observe the P-P intervals. Are they regular (minor variations can be caused by the breath cycle)? If P-P intervals are irregular, is there a pattern?