A referral from someone that you know is always better than online reviews but unfortunately this is not always possible. Before I start engaging with any unknown business I start with a few basic online research actions.
1) Who owns their web domain. Goto https://www.whois.com/whois/ and enter their domain name. Here you can get a good idea to who their domain belongs to and if it is a Nigerian or some other suspect person or company run....
2) Online reviews can also help but look out for companies that they claim use them and do some research on these companies, maybe contact them to confirm that they actually use this services.
3) LinkedIn - check out their employees, address, and any details to match what they supplied to you. Maybe link to someone in the company and ask them a question that might catch them off guard.
4) Company directory - how long have they been in business and does the address match the one they provided to you.
5) Google maps - check their address on street view, do you see their name on the building or does it look like a legitimate place of business?

None of these are fool proof but doing this will give you a good gut feeling.

Ask if you can create an Escrow account for first scope of work, if work is complete as promised founds would be released to them.