It's generally okay to open your kimchi jar to taste it during fermentation, but do so sparingly and quickly to minimize exposure to air and potential contaminants.
Kimchi fermentation relies on anaerobic bacteria (bacteria that thrive in the absence of oxygen). Opening the jar introduces oxygen, which can potentially slow down the fermentation process or encourage the growth of undesirable molds or bacteria. However, a quick taste test won't usually ruin an entire batch. The key is to minimize the amount of time the jar is open and to ensure that any utensils you use to sample the kimchi are clean. After tasting, push the kimchi back down below the brine to maintain an anaerobic environment. The fermentation process will continue as long as the kimchi remains submerged and the proper conditions are maintained. Frequent and prolonged opening of the jar is what you want to avoid.
Use a clean fork or chopstick to sample your kimchi, and avoid double-dipping! Introducing bacteria from your mouth can contaminate the batch and potentially lead to spoilage.