Itching and scratching
Is your dog scratching more than usual? Though there are numerous possible causes for increased scratching in dogs, if you notice it happening more in springtime - and especially if it gets worse after having been outside - it could be allergy-related.
Red, irritated skin
Like humans, dogs can be prone to rashes and other nasty skin conditions and it can often be a tell-tale sign of allergy issues (springtime or otherwise). Check the parts of their body that have the most contact with grass, like bellies and paws.