At the time we are talking about there were 3 types of Reserve Forces: the Army Reserve, made up of ex regulars who had completed the colour part of their service, the Militia who could only be embodied for home defence purposes, and the Rifle Volunteers and Yeomanry. The Rifle Volunteers were trained as infantry and the Yeomanry were trained in the cavalry role. There were also small numbers of gunners and engineers within the Volunteers. The Volunteers and Yeomanry were the reserves which went to reinforce the Regular Army, as formed bodies of men, in times of war or dire threat of war. In contrast, the Army Reserve would be called up as individuals to go into existing Regular units to bring those units up to their war establishment.
All three types of reserve had a training commitment, usually amounting to 2 weeks 'in camp' training. The Volunteers and Yeomanry also had to complete mandatory initial recruit training and annual tests such as shooting and drill, which the Army Reserve did not need to do. The militia were also expected to attend recruit training and an annual camp but they were only committed for 3 years service so their standard of training was generally much lower then the AR or the Rifle Volunteers.
All reservists were paid. I don't have the 1885 figures to hand, but by the turn of the century a 'standard' Army Reservist would be on 3/6d per week (6d per day).
The first class reserve consisted of ex regular soldiers who committed to make themselves available for recall to the colours as and when required, before full mobilisation had been declared. Later they would later be known as the Class A reserve, and later still the 'Ever-Readies'. For this commitment they were paid double the rate for normal Army Reservists. This is why First Class Reserve deserters were treated so seriously: they were effectively taking the money but not complying with their extra commitment. It would usually only be discovered that they had deserted if they failed to come and collect their pay, which was paid in cash, quarterly, usually at the local barracks. However I suspect that since there could have been legitimate reasons for not coming in for your pay on a specified date, such as sickness, there were probably certain other steps to be completed before deciding that someone had deserted.