Can I dismiss a member of staff on suspicion of theft and dishonesty with only word of mouth as evidence? We have proof that money has changed hands but not been put in the till.
You are not the criminal court so you don't need proof beyond reasonable doubt but you do have to have good grounds for believing that, on the balance of probabilities, this employee is responsible. This is a potential gross misconduct situation so dismissal is a possible response.
You appear to have a reasonable belief that there has been a theft so you now need to carry out a reasonable investigation to see how far you can narrow down your suspect list. You can then look at dismissing anyone who you reasonably believe could have carried out the theft.
If following your investigation this employee is the only one left in the frame then you can dismiss him. However, if there is more than one employee left then you would need to consider dismissing everyone who you can't exclude. Just dismissing this employee in those circumstances would be hard to justify.
See also: Employee perks on the rise