You wont be able to, as like I said, its near impossibble to tell how many watts your system is developing under acceleration (surge) without the use of an EagleTree data logger or similiar item. Even then, you may not capture the largest spikes with total accuracy, as it samples the current and voltage (volts x amps = watts) every 10milli-seconds I believe, so you may only catch the start of the spike of the tail end of it.
Typically, I've seen spikes in the 120-130amps region for normal BL setups running 4-6s lipo, so thats somewhere in the 2000-3000watts region. A decent 4s 25c 5000mah lipo can deliver 1800watts, or 6s 2700watts, which is pretty much fine- the hvmaxx being a less powerful system will draw somewhere around 80% of what a 1000watt motor will do. Thats an educated guess though, havent seen many, if any Eagletree sessions carried out on a hvmaxx- might be a question for the novak techs perhaps, though I imagine they will just give you 'on paper' ratings rather than real world test results, which is understandable.
Buy the best batts you can afford at the end of the day.