Subcontracting can be very profitable, especially if you are looking to grow your business. If you constantly get through your own work and have spare days each week/month, virtually all subcontracting work could be mentally classified as 'profit' as it is money you would not normally earned.
Use Experian to carry out credit checks to see if you need to impose credit terms upon them. This may result in you asking for a deposit upfront to secure your services and may help with your cash flow. We certainly do this with all new clients to check their bankability.