Check for free, on your country's official website, whether your car's compulsory insurance is valid.
Free for your own car, with just the registration number: it tells you whether it is recorded as insured on the UK's official insurance database (Motor Insurance Database).
Official website: Motor Insurers’ Bureau
Free for cars in your name, signing in with Cl@ve or a digital certificate: each vehicle's details show whether it is recorded as insured. Spain has no public look-up by plate; for someone else's car (for example after an accident) you ask the Consorcio de Compensación de Seguros (FIVA).
Official website: DGT
Free, with the registration number (no spaces) or the VIN: it shows whether the RCA policy is valid, until when and which insurer issued it.
Official website: AIDA (asigurătorii RCA)
A new or renewed policy can take a few days to show up. If it doesn't and you have the receipt, ask your insurer.
Note in your car's logbook when the insurance runs out and we'll remind you before.