I'm not sure that snipers "work[ed] alone most of the time" - while I know some did, standard doctrine seems to be a minimum of sniper and spotter.

Quoting from Anzac: Gallipoli Marksman by Bob Courtney, curator of weapons and heraldry at the Australian War Memorial, 2001...

Scout marksmen worked in pairs. One observed or ‘spotted’ the terrain while the other shot at the selected target. Both would be good marksmen and they would often change roles of shooter and spotter. The designated pair would usually creep out into no man’s land before dawn and select a position that provided good natural cover and a field of fire. The job was not one for the fidget and patience was of all essence. Once the spotter had selected a target the rifleman would slowly move to take aim. Only one shot was required to improve the tally but it was only credited if the spotter saw the victim fall. An immediate second shot could easily give away their position. It might be hours before another shot was taken. The best one-day tally for Sing was nine confirmed kills.

In his time at Gallipoli, Billy Sing was credited with 150 confirmed kills in regimental records. Major General William Birdwood listed him with a further 51 unconfirmed kills by October 1915, and reportedly even served as Sing's spotter once. Sing supposedly shot a Turk, and remarked to "Birdie" that he wouldn't claim the kill because he missed the fellow at whom he was aiming. Sing's regimental major claimed that Sing had made better than 300 kills, but that's the point of the discussion - there's no proof of the 201 man tally, or the 300+ tally estimated by

One factor that makes Sing's record impressive is that it was achieved over open sights or the primitive Lattey galileean sights (or similar types), not the typical telescopic sight that comes to mind when one thinks "sniper rifle". Germany was issuing telescopic sights during this time frame with the Gewehr-98, but there appears to be no photographic evidence of their presence in Gallipoli.

Getting back to point - snipers tracked their kills by having someone else, usually their spotter, confirm the target was killed. Anything beyond that was speculation, and stated as "unconfirmed" in records and Mentions in Dispatches.

I recently did quite a large amount of work on sniping in the First World War and the issue of ''confirmed kills'' never actually came up in any of the primary and secondary sources. In regards to the First World War though your question breeches the conundrum of what exactly a sniper actually is. It is a term which seems to have been used exceptionally liberally and entails 3 levels of what a sniper could be, following the professionalisation of Sniping by the British Army throughout the war; the first is a sniper removed from company level organisation who has attended a sniping course and falls under the remit of the battalion sniping and intelligence officer (these battalion level snipers became increasingly prominent from 1916 onwards after the amateur beginning of sniping in the First World War), following on from this there were also snipers within individual companies who may have had instruction from an officer who attended a sniping course, and finally there are the general duties riflemen who were encouraged to ''snipe'' whenever it took their fancy. Only battalion snipers can be seen as the elite snipers deployed in a role that we would envisage a stereotypical sniper performing, such as camouflaging themselves and seeking out senior officers instead of taking pot shots through a loophole at fleeting glimpses of movement. This was because battalion snipers were the recruited experts who had been on the sniper courses, whereas company snipers were used to support the battalion snipers. This can be established as the company snipers are rarely mentioned in primary sources such as field manuals, as such it is likely that company snipers differed little from normal sentries or riflemen, who were often encouraged to engage in sniping as a normal trench activity. This is reflected in primary sources such as an article in The Times from May 1915 which is littered with constant references to sniping, which it details as either using a rifle through a loophole or with an attached periscope. This is compounded by a further account which details how the West Kent Regiment, who had recently taken over a trench system, which was dominated by German snipers, were able to turn the balance of control of no-man’s land in their favour. The article attributes this to the officer taking a keen interest in sniping which allowed the West Kents to turn the tide and were coming out on top within four days, probably indicating the use of company level snipers. This level of organisation is probably very similar to the Second World War as after scrapping the sniping courses in the interim period between the wars the only resources the western allies had was the field manuals and accounts from the First World War when sniping courses were re-established.

As such it is exceptionally difficult to identify what exactly we are talking about when discussing sniping as a whole within the First and Second World War. Furthermore although snipers did often work informally in pairs in the First World War it is unlikely that it would be particularly easy to ''confirm'' a kill when firing at fleeting glimpses of movement in a trench or suspected loophole, although snipers on either side often engaged in quite macabre games of indicating misses to one another with a trench shovel. This difficulty is touched upon by Hesketh Heskth-Pritchard, the grandfather of western sniping, who gives an account of inspecting a sniper, who had not attended one of his courses, early in the war who claimed to have shot a German through a loophole, whereas Hesketh-Pritchard who had been observing had seen his shot miss by a good few feet. Thus ''confirming'' a kill is an opportunity which would only really present itself to the elite snipers at Battalion level who would go out camouflaged into no mans land to give themselves a better vantage point. However we are therefore taking them at their word and the process of ''confirming'' a kill seems to not really existed in the First World War.

Sources

The Times, May the 26th 1915

Notes for Infantry Officers on Trench Warfare (London 1916)

The Times, September the 16th 1916

Hesketh Hesketh-Pritchard, Sniping in France: With Notes on the Scientific Training of Scouts, Observers and Snipers (Dutton 1920)

Paddy Griffin, Battle Tactics of the Western Front: the British Armies art of attack 1916-1918 (London 1996)

John Lewis-Stempel, Six Weeks: the short and gallant life of the British officer in the First World War (Orion 2010)

John Terraine, White Heat: the New Warfare 1914-18 (London 1982)

Tony Ashworth, Trench Warfare 1914-1918: the Live and Let Live System (London 1980)