What is the best way to detect the presence of tear gas? I realise that tear gas can be several different things (Pepper spray / CS Gas / CR gas, PAVA, etc)

Is there a good way to detect some or any of these in the air?
Ideally using an electronic sensor that can be reused?
Owlstone make an electronic multi-agent detector - I don't know how much it costs but I would guess somewhere in the range \$10K - \$100K. There might be cheaper single-agent electronic sensors or chemical tests.