I'll start by saying that I can’t provide useful answers for Nazi Germany or Imperial Japan, but I can give some information about some of the ways this was handled in Canada during the Second World War. A lot of this is going to cover official measures meant to address public perceptions and pressure that might be brought to bear on men of "fighting age".

At the time, there was an expectation that men who appeared to be of military age and who appeared to be physically capable of military service would be in the military, and thus would be in uniform. There were some exceptions — men from racialized populations, for instance, were often not wanted in the Canadian military. In some areas of the country, most notably the province of Québec, but also in some other areas, there was also more pronounced public ambivalence about and even outright objection to military service, or at least to some kinds of military service, but that’s its own subject of discussion.

In any event, there was official recognition that these men who were widely expected to be in uniform but who were not actually in uniform would face public repercussions (taunts, insults, etc.). One of the outcomes was the creation of the Applicant for Enlistment Badge, which was issued to:

“Persons who have voluntarily declared their unqualified willingness to serve in and beyond Canada in the Military Forces of Canada, and who are refused enlistment by reason of their not possessing, due to no faults of their own, the necessary qualifications then required for enlistment in the Naval, Army and Air Forces of Canada.”

The full text of the relevant Order in Council (PC 7893 of 11 October 1941) can be found here, and contains a few more details that are worth expanding:

1. The “unqualified willingness to serve in and beyond Canada in the Military Forces of Canada” meant that the recipients had to have volunteered for what was often called “active service” — in other words, volunteering to be sent wherever they might be required, within Canada or overseas.

2. As a result, those who were conscripted under the National Resources Mobilization Act, which for most of the war only meant service within Canada (although the definition of “within Canada” was expanded over time), were not eligible to receive the badge, even if a medical examination during their call-up meant that they were unfit for military service.

3. If someone applied to enlist in the Royal Canadian Air Force or Royal Canadian Navy, both of which were more selective in their enlistment processes, but was turned down because they didn’t meet qualifications, they also had to offer to enlist for active service in the Army and be turned down before they could be eligible for the badge.

4. If someone was ineligible “due to a deformity, disability or a disease which is obvious,” they were not entitled to the badge.

So the Applicant for Enlistment Badge was created to help identify Canadians who were not eligible for military service through no fault of their own, and who appeared to be able-bodied.

If someone had been honorably discharged (for medical or other reasons) after serving in the military and volunteering for “General Service” (the same criteria as mentioned above for the Applicant for Enlistment Badge), they would be eligible for the General Service Badge. If you want the full details, they’re in Order in Council PC 7892, which you can find here.

There had been similar service badges for First World War veterans, so any of them who appeared to be of military age during the Second World War could wear those. They would often be worn for patriotic or other reasons too, of course.

Finally, members of the Merchant Navy could also encounter similar situations, since many of them were not issued uniforms. As a result, the Canadian government also issued a “Canadian Merchant Navy Badge” - full details can be found here. It was meant to serve much the same purpose in identifying people who might otherwise give the appearance of being civilians not in service.

All of these Second World War badges were introduced in 1941, but were backdated to September 1939. There were requirements that people wearing the badges also carry with them a certificate establishing their entitlement to wear it. The penalty could be up to 500 Canadian dollars (approximately 8,000 Canadian dollars today, based on inflation), or six months' imprisonment.

Anyone reading this far will have noted that eligibility for these badges (except the Merchant Navy badge) depended on the applicant having volunteered for “General Service”, which meant, as mentioned before, that conscripts were not eligible. This was part of a larger campaign to pressure Canadians into volunteering for general service, rather than serving as conscripts, but that’s a larger story.

I will note that even if someone was in uniform as a result of conscription, there would still be pressure to “go active” (as it was often called) by volunteering for General Service. In the case of the navy and the Air Force, "going active" was a given part of enlisting, but since conscripts wound up in the army, there was significant pressure brought to bear on them to "go active". Some of these measures included visible recognition: a “GS” (for “General Service”) badge, and the creation of the Canadian Volunteer Service Medal. There were other measures and efforts too, some of them quite coercive, but that’s better suited to its own separate discussion.

I will also note that the language in all of these Orders in Council specified "persons". This meant that women were eligible for the badges mentioned above, including the Applicant for Enlistment Badge and the Merchant Navy Badge, although the public pressure and expectations for Canadian women to be in uniform during the war would not have been as great because of prevailing attitudes at the time. Indeed, the women's branches of the armed services did have to contend with some parts of the public who considered women unsuitable for military service, or who thought that military service would have a detrimental effect on women who joined up. Again, that's also probably better suited for its own discussion with someone who is better acquainted with the literature than I am.

For sources, I can suggest Daniel Byers’ Zombie Army: The Canadian Army and Conscription in the Second World War, for a specific focus on how the Canadian Army handled this issue. There’s a broader literature on conscription and human resource mobilization in Canada, including Michael D. Stevenson’s Canada's Greatest Wartime Muddle: National Selective Service and the Mobilization of Human Resources during World War II, and some service-specific studies of enlistment, such as Allan English’s The Cream of the Crop: Canadian Aircrew, 1939-1945. Some general home front surveys such as Jeff Keshen’s Saints, Sinners, and Soldiers: Canada's Second World War are also worth a look.

I would also suggest taking a look at Scott Thompson’s “Real Canadians: Exclusion, Participation, Belonging, and Male Military Mobilization in Wartime Canada, 1939–45,” Journal of Canadian Studies, Volume 50 Issue 3, Fall 2016, pp. 691-726, but will note that it is behind a paywall.

In any case, this may be more than you were looking for, and with luck other contributors will chip in on the situation in other countries.

Hope this helps!