Vents are a good thumbnail to start with. If you plant out a live tank, the lighting will elevate your tank temps about 5 degrees on average so a heater will not be necessary. A night time drop of ten degrees is acceptable for vents and other dart frogs. The real danger for dart frogs comes with higher temps. Ventrimaculatus especially being a lowland species, don't do very well with temps over 78 degrees. Maintain tank temps between 65-75 and you should be just fine. Humidity over 80%.

Sexing thumbnails isn't easy. Generally calling determines 100%male. Egg laying determines 100% certainty of female. Some accurate educated guesses can be made by body shape and size ( female gravid ) but 100%sex certainty is determined that way. Larger mature frogs can usually be sexed by experienced breeders and hobbyists.