When I was trip planner / advisor for an adventure group I used the following methods (I also found much of the method useful to determine which direction to hike on loop hikes):
Load the trail into a topo application and observe the trail's profile. Calculate the elevation gained and lost for the hike (most do this automatically).
For a typical day hike this might be 500-1,000 feet of elevation change once you count all the uphills and downhills for even relatively easy terrain.
For loop trails and determining direction, look for steep uphills or downhills and determine your groups abilities as well as where it will fall in timing. Some people with knee problems have major issues doing down steep slopes, others find the uphills killer (usually in mixed groups that have people packing heavier weights). Also, sometimes it's far wiser to handle a large climb early on on a multi-day and end with a relatively easy route after that.
The easiest way of telling the steepness of a hike is it's grade, again the software can usually do this for you. (Grade = elevation/distance x 100) To give a point of reference on grade: A typical road is generally less than 6%. Between 6% and 8% it's usually signed in the US, if the hill is of any significant length, to warn trucks. 8% to about 11% is extremely rare on anything except mountain switch backs.
In addition, calculate things such as soil type (sandy and loose stuff makes it harder) as well as elevation, weather and other terrain important issues. Anything above 8,000 feet in elevation is going to be significantly harder on a lowlander even if it's a completely flat trail.
As an example, lets take a hike to Crystal lake in Rocky Mountain National Park. It's a typical out and back but for the sake of calculation let's just figure it going one way. (all numbers from Delorme Topo USA and may be rounded a bit)
Start Elevation: 8,600 ft
End Elevation: 10,300 ft
Distance: 7.3 miles (38,544 feet)
Climb Elevation: 3,572 ft
Desc Elevation: 616 ft
Average Grade as 11%
From looking at the above, you can see that it doesn't have much of any downs hill, but is instead an uphill trudge most of the way. With those conditions I'd suggest only fit hikers attempt the trail, especially due to the higher elevation so it would receive a rating of moderate/strenuous.
Now on the other hand, if we go to the trail to the south the Beaver Mountain Trail (a loop trail) has the following stats:
Start/End Elevation: 8,900 ft
Distance: 6.8 Miles
Climb/Desc Elevation: 1,347 ft
Max Elevation: 9,247 ft
Min Elevation: 8,434 ft
Average Grade: 9%
Overall, this trail has far less climb / desc elevation change. At 1,347 ft and average grade of 9% that'll put it in the range of yes there are some up and down hills, but they are less steep and it's overall lower altitude combine to make it a milder trip than Crystal Lake. It also has about as much uphill as downhill as compared to a solid uphill slog. However, with a peak elevation of over 9,000 ft elevation acclimation for a few days before starting is recommended to help reduce the affects of altitude.
For those wanting an example with a little less elevation, take the Paddy Creek Wilderness Trail in Missouri. It is again a loop so obviously it will have the same elevation for start and finish.
Start/End Elevation: 1,255 ft
Distance: 14.5 Miles
Climb/Desc Elevation: 2,300 ft
Max Elevation: 1,290 ft
Min Elevation: 889 ft
Average Grade: 6%
So here we've got a longer train, so the climb / desc elevations will be larger numbers. But, take a glance at the 6% grade, so you know that overall, the hills are mild. But, take a close look at your min and max elevation, they are only 400ft different. Basically, to get that much climb and desc without much change in overall elevation you are currently looking at a very "lumpy" trail, so there will be almost no flat spots but instead it's up/down/up the whole trail. On the plus side, the low altitude and relatively mild terrain makes that something I'd put on the easy side of moderate for a typical hiker. If it wasn't for the constant ups and downs it'd easily be rated an "easy".