Imaam Abu ‘Uthmaan Ismaa’eel as-Saaboonee (rahimahullaah died 449H) said:
“One of the distinguishing signs of ahlus-Sunnah is their love of the Imaams of the Sunnah, and its Scholars and its helpers and its allies – and likewise their hatred for the Imaams of bid’ah, those who invite to the Hellfire.” 
When innovations became widespread, and the adherents to the Sunnah became few in number, and ahlul-bid’ah claimed they were upon the truth, and would hide their deviation, ahlus-Sunnah would test those who caused them doubt.
Sufyaan ath-Thawree (rahimahullaah died 161H) said:
“Test the people of Mawsil (in Iraaq) with Mu’aafaa ibn ‘Imraan.” 
Abdur-Rahmaan ibn Mahdee (rahimahullaah) said:
“The people of Basrah with Ibn `Awn, so if you see a man loving him, then be at ease with him. For the people of Koofah, it is Maalik ibn Mighwal and Zaa’idah ibn Qudaamah, so if you find a man loving him, have hope in his goodness. As for the people of Shaam (Syria, Jordan, etc), it is al-Awzaa’ee (died 157H) and Abu Is-haaq al-Fazaaree. For the people of Hijaaz, it is Maalik ibn Anas (died 179H).” 
Ahmad ibn ‘Abdullaah ibn Yoonus (rahimahullaah) said:
“Test the people of Mawsil with Mu’aafah ibn ‘Imraan, and if they love him then they are ahlus-Sunnah, and if they hate him then they are ahlul-bid’ah – just as the people of Koofah should be tested with Yahyaa.” 
Ibn Mahdee (rahimahullaah) said:
“If you see a person of Basrah loving Hammaad ibn Zaid, then he is a person of Sunnah.” 
Qutaibah ibn Sa`eed (rahimahullaah) said:
“If you see a man loving the People of Hadeeth such as: Yahyaa Ibn Sa’eed, ‘Abdur-Rahmaan Ibn Mahdee, Ahmad Ibn Muhammad Ibn Hanbal, Ishaaq Ibn Raahawaih, then he is upon the Sunnah. And the one who opposes them, then know he is an innovator (mubtadi’).” 
Benefits from the book, Lammud-Durr al-Manthoor min al-Qawlil-Ma’thoor, of Jamaal al-Haarithee.
And in these times we say: If you find a person loving Ibn Baaz, al-Albaanee, Ibn Uthaimeen, the great scholars of this era (rahimahumullaah). And likewise, Saalih al-Fawzaan, Ahmad an-Najmee (rahimahullaah), Rabee’ ibn Haadee, Muhammad Amaan al-Jaamee (rahimahullaah), Muqbil bin Haadee (rahimahullaah), Zayd al-Madkhalee, Ubaid al-Jaabiree and their likes, then know he is a man of Sunnah, for loving them is a distinguishing sign of ahlus-Sunnah. And having hatred and enmity for them is a distinguishing sign of ahlul-bid’ah. 
 ‘Aqeedatus-Salaf As-haab al-Hadeeth (171)
 Tahdheeb al-Kamaal (28/153), Siyar (9/82), at-Tahdheeb (10/181)
 al-Laalikaa’ee (41), Tareekh Dimashq (7/128)
 al-Laalikaa’ee (58)
 al-Jarh wat-Ta`deel of Ibn Abee Haatim (1/183), al-Laalikaa’ee (38)
 al-Laalikaa’ee (59), Sharaf As-haab al-Hadeeth (143)
 See Sharh ‘Aqeedat us-Salaf As-haab al-Hadeeth, (p.347) of Shaikh Rabee’ for the permissibility of such a comment.