!rant
When applying for a new job, how do you tell the difference between somewhere that is really old fashioned but wants someone come in and make improvements to processes and coding standards... And somewhere that pretends to want those things, but actually has no intention of letting anyone make those changes?

Research the company a bit. Find people who work there and ask them .
Edit1 : Don't under any circumstance ask anyone from HR about that . They might not be the most truthful since they're looking to sell their company .

@adracea Not sure if this is an advantage or not, but they currently only have 2 full time devs and a manager that dabbles - both of which have been there for 15 years so they admit that they are behind the times...

But then, how comfortable are they with someone actually coming in and making sweeping changes? I've been promised the ability to make changes before (admittedly at much bigger companies) and I was forced to move to keep my knowledge up to date with modern standards

From what country are you from? In Austria you have one month of "trial", during that month both parties can terminate the contract without a reason or cancellation period.

Just find out right away and quit when necessary

@slaat But then there is the uncertainty period of being jobless and looking for work again so quickly? We have probation periods here - were it is easy for both parties to terminate a contract

First question should be, what attempts have they made in the past?

Are they using git or mercurial? If they aren't, that should raise a red flag.

If they have two developers who have been there 15 years, you would probably meet a lot of resistance no matter what changes you propose.

During interviews I make it a point to ask the devs or managers very pointed questions about what I'm looking to know and I try to determine what they are not telling me and what they are "stepping" around with their words.

I ask directly. I would just say 'How resistant are staff or stakeholders to change? Will I spend all my time here convincing people that change is needed or will people generally play ball'

Deffo don't believe everything HR tells u. That's from experience. Have a chat with a developer and ask what's the legacy code like and what new tech have they recently started using, frameworks/tools/utilities/process such as linting and testing improvements... You should be able to judge what it's like with that info.