Russia started to actively deploy armored vehicles to Donbas in June 2014. When Girkin's squads were breaking out of encircled Sloviansk on July 5, 2014 they were using Russian military equipment. As spotted on this video from Kramatorsk, they were moving around in an armored personnel carrier BMP-2 labeled "To Kyiv", "To Lviv," and marked with no. 125.
There are photos of the same BMP-2, yet without "To Kyiv" label, but having the same green camouflage, as well as no. 125 printed at the same place, with the same font and having the same outstanding characteristics (characters "1" and "2" in the rear part are skewed down and to the left, stand a bit aside of character "5"), which were posted in social media by servicemen of the Armed Forces of Russian Federation (AF RF) - drivers of military tractors Artem Lentsa and Igor Ivanov.
Saved copy of a VKontakte (Russian social media, analogical to Facebook) page: https://archive.is/Q8Ajw
Photo original https://pp.vk.me/c617121/v617121211/1e1
A page with this photo has been deleted, link to original: http://ic.pics.livejournal.com/sled_vza
Comparative analysis of the two infantry carriers on the photos:
Most of the light armored vehicles deployed by Russia to the Donbas, including this this BMP-2, are old Soviet models: infantry carriers BMP-1, BMP-2, non-modernized APCs BTR-80 and MT-LB, airborne infantry fighting vehicles BMD-2. As long as the same models are also in service of the Ukrainian army, it makes it really difficult to reveal Russian involvement in all the cases. However, in some events, as with BMP-2 "Lavina", deployment of examples with well-recognized outstanding characteristics is usually well-monitored. The Russian origin of some old models of armored vehicles is revealed by tactical insignia of AF of RF, when they were not removed fully due to negligence of those on duty.
APCs BTR-80 with tactical insignia of the 23rd independent motorized rifle brigade (m/u 65439, Samara) on the Donbas.
To compare, here is the same insignia on military vehicles of the 23rd independent motorized rifle brigade.
To compare, this is the same insignia on military vehicles of the 11th independent engineering brigade of the AF of RF.
This non-modernized BTR-80 was spotted both in the 11th independent engineering brigade of the AF of RF in Kamensk-Shakhtinskiy in 2013 and in the Donbas in summer of 2014. Taking into account a different camouflage and the second tactical insignia, which is used by the 18th independent motor-rifle brigade, we can assume that this APC was deployed from the 18th brigade to Kamensk-Shakhtinskiy after reinforcement by later model - BTR-82AM.
Photo original: https://pp.vk.me/c620920/v620920990/fad
Comparison by camouflage, tactical insignia, and a form of the stain of a removed insignia
This is the same BTR-80 (same camouflage, the black stain, the red tactical insignia "201", same rust under "0" in the number) that was shot on video in July 2014 in a convoy of military vehicles in the Rostov region near the town of Shakhty. The non-modernized BMD-2 in possession of separatists in the Donbas:
Insignia is not painted over fully, and under the newly-painted star there is still a tactical sign - white circle - which can be seen with a white line. The above-mentioned driver of a Russian military tractor Artem Lentsa, who was redeploying the BMP-2 (no. 125) to the separatists, also made photos with a similar airborne infantry fighting vehicle - also non-modernized, with analogical tactical insignia, camouflage pattern, numerical combination:
Saved VK page https://archive.is/c4e1b
Original of the photo: https://pp.vk.me/c617121/v617121211/1e1dd/2a1ZgdBdpqE.jpg
Another saved VK page: https://archive.is/4fYAy
Airborne infantry fighting vehicles of the same modification, with the same tactical insignia, camouflage pattern, numerical order can be spotted in the 108th airborne-assault regiment of the 7th airborne-assault division of Novorossiysk:
It is much easier to deal with the modern models of Russian armored vehicles. The very fact of them being spotted in the Donbas, without need to provide photo- or video-evidence of an exact vehicle on both sides of the border, is already a proof of Russian military aggression against Ukraine. Fighting in Ukraine with these modern military vehicles is more common for professional army squads of the Armed Forces of Russian Federation. On Aug. 16, 2014, a number of videos were published on YouTube, all filmed near the village of Dmytrivka (Shakhtarskyi district, the Donetsk region). They show BTR-82AM armored infantry carriers and armed people of Caucasian appearance without insignia.
On the nearest BTR-82AM white triangles painted on the armor and on the cover of the spotlight can be seen:
Quite expectedly, pro-Russian websites called the people on this video "Caucasian volunteers". However, in January 2015 VKontakte user Ildar Bagalov (who served in military unit #27777, the 18th independent motor rifle brigade, Chechnya), in his album "Contract First Combat Task!! Won't forget" uploaded photos geo-tagged in Kuybyshevskiy district of the Rostov region, near the village Dmytrivka.
What is shown on the photos is quite telling: Russian military with no insignia on their uniform on BTR-82AM APCs with white triangles painted on their armor:
Saved copy of a VK page: https://archive.is/Kjodt
Original of the photo: https://pp.vk.me/c621321/v621321089/87a3/CmcC2q63ZNU.jpg
The same white triangle on armor can be spotted on photos of Rolan Ramazanov - servicemen of the Russian Army. Rolan can be found on group photos of Anton Tumanov of the same 18th independent motor rifle brigade, who died near Snizhne.
Thus, it is obvious that the BTR-82AM APCs filmed near Dymytrivka in August 2014 were possessed by the 18th independent motor rifle brigade of Chechnya, which used white triangle insignia on their armor when intervening the Donbas. Russian BTR-82AM APCs were also used in the Luhansk region. One of those was destroyed in the village of Novosvitlivka. Due to Russian First channel's ignorance, this burnt Russian APC was shown in their news piece of Sept. 6, 2014.
Later, better quality photos were also published in social media: