the classic way is telling the intensity is through house/building damage. but it's hard to tell sometimes so below is a list.

EF0's cause very minor damage and objects are tossed.
EF1's cause minor damage, roofs and fences may be damaged
EF2's cause moderate damage, cars moved and roofs torn of.
EF3's cause strong damage almost destroying houses.
EF4's cause very strong damage, houses are destroyed and car are thrown around like paper.
EF5's cause intense damage, foundations are ripped from the ground.

the cons are that if a tornado doesn't strike a building then it's hard to tell.

measuring funnel sizes are another way of telling what strength a tornado is. apparently many people estimate strengths by size and assume that is the strength like a big tornado would be an EF5 right?
WRONG

[an EF3 striking corwan]
tornados are measured by wind speed and damage they do but in storm chasers tornado are also measured by size like this EF3 tornado. (above) its an average EF3 size. so thats another method to measure tornadoes.

the cons are they may change this in a future update or it may be a different shape then the strength. (NOTE: littlle cloud games are dead so they wont update(EXTRA NOTE: they arent anymore!))

[EF4 roaming a field]
this is the way i can tell EF strengths very quickly. strength bonuses often multiply your score/XP
by a lot. here is a list.

x1 = EF0
x1.1 = EF1
x1.2 = EF2
x1.3 = EF3
x1.4 = EF4
x1.5 = EF5

they are found when you take a picture with your camera at the bottom of your score list.

the cons are this will change in the new update, you cant tell until it does damage

[radar of an another EF4 tornado]
one method is to rely on the radar. apparently strong tornadoes (EF4>) have yellow boxes around the red box, if the red box is on it's own then it's a EF3<.
cons are that this is not always the case.

another method is to measure wind speed with you wind gauge, but this is really hard to without getting too close.

or you know drive into the tornado.