You can tell the grade of a trail by doing Route To / Route From and selecting the walking option. After computing the route it shows you a fairly accurate elevation profile, as well as the length and total elevation gains/losses.
If you're using imperial units, you can also quickly estimate difficulty by doing the following:
- take the total elevation change, in hundreds of feet, eg 900ft = 9 * 100ft
- take the total distance, in tenths of a mile, eg 2.1 mi = 21 * .1 mi
- divide the elevation by the length and convert to a percentage: 9/21 ~ 43%
- grades are like so:
0-40%: relatively easy
40-70%: moderately big elevation, may be hard to sustain
70-100%: steep terrain, may involve some scrambling
>100%: very steep, technical terrain. Sometimes involves ladders or a static line in particularly steep sections
For example, there's a pretty lengthy trail near me called Shining Creek, that's very sustained, but I wouldn't call it steep. It's got 2300ft of elevation change over 3.9mi, which corresponds to a 58% steepness, aka the upper end of a moderately difficulty hike if sustained.