I agree with idefix: temperature layer at 850hPa is useful. Looking at winds also helps: wind abruptly changes direction by maybe 45 degrees at the front. Over ocean, surface winds are fine, but over land I'd look at winds at 850-900hPA as well. It also helps to look at precipitation: it rains there. It also helps to see the pressure contours: they have a little kink at the frontal lines. My favorite view at windy.com for that is: color as precipitation, pressure contours, and particle wind animation.
There are some related discussion here https://www.morganscloud.com/2017/06/28/grib-weather-files-you-gotta-see-the-rain/