StackExchange sites have a long history of dealing with questions that don't fit the format of having "only one correct answer."
The problems, in short, are:
- Asking for a list of solutions means multiple answers may be correct, and thus difficult to determine the best answer.
- Whenever multiple items are involved, there are inevitable duplicates from provided answers, making more "noise" and degrading the overall quality.
- List-type questions also suffer from having suggested answers becoming obsolete or incorrect at different future dates, meaning increased maintenance of the answers is required.
Personally, I have thought list-type questions work in the SE context, because if each person answering suggests one item, then the votes speak very well for the community-acceptance or approval of each answer. However, the problems listed above still apply, and so generally list-type questions are frowned upon.
So, in the case of your specific question, you would be better off finding one or two existing technologies and asking about specific characteristics. For example, if you had discovered Passerby's suggestion of detecting UV inks, you might ask a question about it specifically. "What type of sensor can I use to detect UV reflection in printed ink?" Except this looks very close to a shopping question, which is another type of question that is frowned upon.
This site expects a fair amount of prior research, and as such, it would be even better if you found a sensor that you thought would work, and perhaps even drafted a rough idea of your implementation of it. Say you had an opamp and a phototransistor and you weren't getting a strong enough signal from it. It would be extremely welcome to post a partial schematic and perhaps show some real or calculated figures.
In summary, EE.SE gets a lot of questions that are about the initial planning stages of products and prototypes, but unfortunately there are just too many variables and options for the site to be helpful in a brainstorming or research capacity. A forum is more suited to the sort of back-and-forth that is needed between engineers when designing something, and SE is not a forum.
(See also: Should "how to design a project" questions always be put on hold as "too broad"?)