Short of asking the supplier or measuring it yourself, you can't. But most standard 510 atomisers are somewhere around the 2.2-2.4 ohm range. 510s do not tend to come higher than 2.4 ohms unless they are high voltage atomisers (much higher resistance) that are designed only for high voltage usage (that many people think defeats the whole purpose of 5+ voltage usage anyway).
LR atomiser general rule of thumb
The lower the resistance an atomiser goes the greater the throat hit and vapour production, but this is often at the expense of flavour.
The lowest atomiser resistance is 1.5 ohm approx.
But many people find these too harsh, and wrecks the flavour.
And so, people who want to go lower, may opt for a 2 ohm resistance instead, hereby not sacrificing the flavour reproduction.
In fact some 2 ohm atomisers can have better flavour than ones @ 2.6, depending on type of atomiser.
Each type of atomiser has different qualities.
Brief guide to atomisers
510 - good all rounder for throat hit, vapour and flavour.
901 - better for flavour.
801/302 - somewhere in between the 510 and 901, good on vapour production, but with different sort of airier action...
M series (M301/M601) - also somewhere in between, but many think is better than the 510 in all aspects...
306 - great for flavour and vapour in the LR version, dripping only (the standard resistance version is not so prevalent these days, mainly due to the tiny cartridge capacity).
eGo Tank atomisers, very convenient (in respect to not having to keep topping up, just use the Tank until it runs out, then refill), but quite muted flavour and not particularly great throat hit.
All these will also vary (maybe quite considerably) between brand/manufacturer.
